fix: missing Javadoc heading

Author: ppkarwasz

log4j-core/src/main/java/org/apache/logging/log4j/core/pattern/NamedInstantPattern.java

@@ -120,7 +120,7 @@ public String getPattern() {
      *     </tr>
      *   </tbody>
      * </table>
-     * <h2></h2>
+     * <h4>Rationale</h4>
      * <ul>
      *   <li>
      *     <p>