Fix incorrect HTML headings in Javadoc (#2200)

extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

@@ -119,7 +119,7 @@ import java.util.Map;
  *       .registerSubtype(Diamond.class);
  * }</pre>
  *
- * <h3>Serialization and deserialization</h3>
+ * <h2>Serialization and deserialization</h2>
  * In order to serialize and deserialize a polymorphic object,
  * you must specify the base type explicitly.
  * <pre>   {@code
@@ -158,7 +158,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
   public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName, boolean maintainType) {
     return new RuntimeTypeAdapterFactory<>(baseType, typeFieldName, maintainType);
   }
-  
+
   /**
    * Creates a new runtime type adapter using for {@code baseType} using {@code
    * typeFieldName} as the type field name. Type field names are case sensitive.
@@ -244,7 +244,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
         } else {
             labelJsonElement = jsonElement.getAsJsonObject().remove(typeFieldName);
         }
-        
+
         if (labelJsonElement == null) {
           throw new JsonParseException("cannot deserialize " + baseType
               + " because it does not define a field named " + typeFieldName);
@@ -282,7 +282,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
               + " because it already defines a field named " + typeFieldName);
         }
         clone.add(typeFieldName, new JsonPrimitive(label));
-        
+
         for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
           clone.add(e.getKey(), e.getValue());
         }

gson/src/main/java/com/google/gson/TypeAdapter.java

@@ -96,7 +96,7 @@ import java.io.Writer;
  */
 // non-Javadoc:
 //
-// <h3>JSON Conversion</h3>
+// <h2>JSON Conversion</h2>
 // <p>A type adapter registered with Gson is automatically invoked while serializing
 // or deserializing JSON. However, you can also use type adapters directly to serialize
 // and deserialize JSON. Here is an example for deserialization: <pre>   {@code

gson/src/main/java/com/google/gson/TypeAdapterFactory.java

@@ -22,7 +22,8 @@ import com.google.gson.reflect.TypeToken;
  * Creates type adapters for set of related types. Type adapter factories are
  * most useful when several types share similar structure in their JSON form.
  *
- * <h2>Example: Converting enums to lowercase</h2>
+ * <h2>Examples</h2>
+ * <h3>Example: Converting enums to lowercase</h3>
  * In this example, we implement a factory that creates type adapters for all
  * enums. The type adapters will write enums in lowercase, despite the fact
  * that they're defined in {@code CONSTANT_CASE} in the corresponding Java
@@ -90,7 +91,7 @@ import com.google.gson.reflect.TypeToken;
  * If multiple factories support the same type, the factory registered earlier
  * takes precedence.
  *
- * <h3>Example: composing other type adapters</h3>
+ * <h3>Example: Composing other type adapters</h3>
  * In this example we implement a factory for Guava's {@code Multiset}
  * collection type. The factory can be used to create type adapters for
  * multisets of any element type: the type adapter for {@code

gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java

@@ -40,7 +40,7 @@ import java.util.Map;
 /**
  * Adapts maps to either JSON objects or JSON arrays.
  *
- * <h3>Maps as JSON objects</h3>
+ * <h2>Maps as JSON objects</h2>
  * For primitive keys or when complex map key serialization is not enabled, this
  * converts Java {@link Map Maps} to JSON Objects. This requires that map keys
  * can be serialized as strings; this is insufficient for some key types. For
@@ -65,7 +65,7 @@ import java.util.Map;
  *   at com.google.gson.ObjectNavigator.navigateClassFields
  *   ...</pre>
  *
- * <h3>Maps as JSON arrays</h3>
+ * <h2>Maps as JSON arrays</h2>
  * An alternative approach taken by this type adapter when it is required and
  * complex map key serialization is enabled is to encode maps as arrays of map
  * entries. Each map entry is a two element array containing a key and a value.

gson/src/main/java/com/google/gson/stream/JsonReader.java

@@ -62,7 +62,7 @@ import java.util.Objects;
  * Null literals can be consumed using either {@link #nextNull()} or {@link
  * #skipValue()}.
  *
- * <h3>Example</h3>
+ * <h2>Example</h2>
  * Suppose we'd like to parse a stream of messages such as the following: <pre> {@code
  * [
  *   {
@@ -161,7 +161,7 @@ import java.util.Objects;
  *     return new User(username, followersCount);
  *   }}</pre>
  *
- * <h3>Number Handling</h3>
+ * <h2>Number Handling</h2>
  * This reader permits numeric values to be read as strings and string values to
  * be read as numbers. For example, both elements of the JSON array {@code
  * [1, "1"]} may be read using either {@link #nextInt} or {@link #nextString}.
@@ -171,7 +171,7 @@ import java.util.Objects;
  * precision loss, extremely large values should be written and read as strings
  * in JSON.
  *
- * <h3 id="nonexecuteprefix">Non-Execute Prefix</h3>
+ * <h2 id="nonexecuteprefix">Non-Execute Prefix</h2>
  * Web servers that serve private data using JSON may be vulnerable to <a
  * href="http://en.wikipedia.org/wiki/JSON#Cross-site_request_forgery">Cross-site
  * request forgery</a> attacks. In such an attack, a malicious site gains access

gson/src/main/java/com/google/gson/stream/JsonWriter.java

@@ -58,7 +58,7 @@ import java.util.regex.Pattern;
  *       Finally close the object using {@link #endObject()}.
  * </ul>
  *
- * <h3>Example</h3>
+ * <h2>Example</h2>
  * Suppose we'd like to encode a stream of messages such as the following: <pre> {@code
  * [
  *   {