Nagios module pillar docs ex update

When copy/pasting the example nagios check Pillar shown in the
[online](http://docs.saltstack.com/en/latest/ref/modules/all/salt.module
s.nagios.html) and sys.doc documentation the load check doesn't run. By
adding the spaces the check is then run. It seems like there should
always be a space to avoid this from happening.
Merge pull request #18405 from tbaschak/nagios-pillar-example

Nagios module pillar docs ex update