Remove a mention of "rvm help" that is unnecessary

This sentence doesn't really make much sense with "rvm help mount" at
the end. None of the other actions explicitly mention "rvm help" in
their short descriptions because there's already a mention at the top
that "rvm help action-name" may provide more information, and "mount"
appears in the list of commands available with rvm help.
Merge pull request #2573 from carols10cents/cleanup_readme

Remove a mention of "rvm help" that is unnecessary