p/ping_: honor ping env variable

As stated by Andreas Schamanek,

	The following environment variables are used by this plugin
		ping            - Ping program to use

	However, ping_ doesn't use that environment var, as it finally runs :

		${PING:-ping} ...

This commit fixes the typo in that last line

Closes: D#769415
Merge branch 'bts/769415-ping_-plugin-does-not-honor-ping-env-variable' into stable-2.0

Conflicts:
	plugins/node.d/ping_.in