Small addition to usage doc for "list"

Provides a clue which command to look at to get a list of all available rubies. Aligns the output of "rvm usage" with "rvm help list".

I've found the usage doc confusing because it doesn't provide a clue for how to get a list of available rubies. IMO, the core basic use case for rvm is:

1. See what rubies are available
2. Install one

The usage doc has a lot of info about #2, but not about #1. This pull request is a small change in that direction.