Remove a mention of "rvm help" that is unnecessary

This change was small enough that I felt comfortable just opening a PR to start with ;)

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.