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.
Merge pull request #113 from cdepillabout/devel

Fix ping_ plugin's documentation to use _warning.