docs: don't suggest './rbt status'. That fails. Instead, './rbt help'

For the background, see this mail thread
https://groups.google.com/forum/#!topic/pants-devel/oZvsn_LzDNU

Testing Done:
moved my cached cookie aside
ran ./rbt commands
This review posted OK.

Bugs closed: https://github.com/pantsbuild/pants/issues/136

Reviewed at https://rbcommons.com/s/twitter/r/384/