RFR: 8239581: Improve javadoc example for @jdk.jfr.Category

[Erik Gahlin]

Hi,

Could I have a review of a javadoc improvement for the Category annotation.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8239581

Webrev:
http://cr.openjdk.java.net/~egahlin/8239581/

Thanks
Erik