Fix ping_ plugin's documentation to use _warning.

The ping_ pluging recommended to use env.ping_warn and env.ping_crit in
/etc/munin/plugin-conf.d/conffile, but really it should be
env.ping_warning and env.ping_critical.

The plugin.sh file only looks for *_warning and *_critical, not *_warn
and *_crit.


This fix is for this ticket:
http://munin-monitoring.org/ticket/1353