fix headings in java.sql.rowset

The following issues are reported regarding headings in documentation comments.
For a general background information, see this message to jdk-dev@ojn.
   https://mail.openjdk.java.net/pipermail/jdk-dev/2019-March/002671.html



open/src/java.sql.rowset/share/classes/javax/sql/rowset/spi/SyncFactory.java:62: Note: missing heading <h2>
 * <h3>1.0 Overview</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/spi/SyncProvider.java:58: Note: missing heading <h2>
 * <h3>1.0 Naming Convention for Implementations</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/Joinable.java:31: Note: missing heading <h2>
 * <h3>1.0 Background</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/JoinRowSet.java:51: Note: missing heading <h2>
 * <h3>1.0 Overview</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/WebRowSet.java:39: Note: missing heading <h2>
 * <h3>1.0 Overview</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/Predicate.java:35: Note: missing heading <h2>
 * <h3>1.0 Background</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/JdbcRowSet.java:39: Note: missing heading <h2>
 * <h3>1.0 Overview</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/CachedRowSet.java:71: Note: missing heading <h2>
 * <h3>1.0 Creating a <code>CachedRowSet</code> Object</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialClob.java:47: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialStruct.java:54: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialRef.java:40: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialJavaObject.java:50: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialDatalink.java:46: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialBlob.java:54: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/SerialArray.java:53: Note: missing heading <h2>
 * <h3> Thread safety </h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/BaseRowSet.java:42: Note: missing heading <h2>
 * <h3>1.0 Overview</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/FilteredRowSet.java:41: Note: missing heading <h2>
 * <h3>1.0 Background</h3>
   ^
open/src/java.sql.rowset/share/classes/com/sun/rowset/providers/RIXMLProvider.java:40: Note: missing heading <h2>
 * <h3>1.0 Background</h3>
   ^
open/src/java.sql.rowset/share/classes/com/sun/rowset/providers/RIOptimisticProvider.java:40: Note: missing heading <h2>
 * <h3>1.0 Backgroud</h3>
   ^
open/src/java.sql.rowset/share/classes/javax/sql/rowset/serial/package-info.java:37: Note: missing heading <h2>
 * <h3>1.0 SerialArray</h3>
   ^
open/src/java.sql.rowset/share/classes/com/sun/rowset/providers/package-info.java:36: Note: missing heading <h2>
 * <h3>1.0 <code>SyncProvider</code> Reference Implementations</h3>
   ^
open/src/java.sql.rowset/share/classes/com/sun/rowset/package-info.java:33: Note: missing heading <h2>
 * <h3>1.0 Available JDBC RowSet Reference Implementations </h3>
   ^