RFR: XXS JDK-8221997: fix headings in jdk.javadoc

[Jonathan Gibbons]

Please review a small docs fix to close up a hole in the series of 
headings in

src/jdk.javadoc/share/classes/jdk/javadoc/doclet/package-info.java

The number in all headings has been reduced by 1.

Before the change:

$ grep 'h[1-6]' 
src/jdk.javadoc/share/classes/jdk/javadoc/doclet/package-info.java
  * <h3>Terminology</h3>
  * <h3>Options</h3>
  * <h4>Interactions with older options.</h4>
  * <h3>Example</h3>
  * <h3><a id="migration">Migration Guide</a></h3>

After the change:

$ grep 'h[1-6]' 
src/jdk.javadoc/share/classes/jdk/javadoc/doclet/package-info.java
  * <h2>Terminology</h2>
  * <h2>Options</h2>
  * <h3>Interactions with older options.</h3>
  * <h2>Example</h2>
  * <h2><a id="migration">Migration Guide</a></h2>

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8221997
Webrev: http://cr.openjdk.java.net/~jjg/8221997/webrev.00/