RFR (JDK 13) 8220254: fix headings in java.xml

[Joe Wang]

Please review a patch for the wrong headings. This patch passed build 
with the new doclint enabled and doccheck (thanks Jon!)

JBS: https://bugs.openjdk.java.net/browse/JDK-8220254
webrev: http://cr.openjdk.java.net/~joehw/jdk13/8220254/webrev/

Thanks,
Joe