Modify a typo in monitoring.md

As I mentioned in SPARK-1765, there is a word 'JXM' in monitoring.md.
I think it's typo for 'JMX'.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #698 from sarutak/SPARK-1765 and squashes the following commits:

bae9843 [Kousuke Saruta] modified a typoe in monitoring.md