Fix wording of 'bin/remove' prompt for repositories

Summary: This UI recommends `bin/remove destroy X`, but should recommend `bin/remove destroy rX` (with `r`), because the remove script now takes any object monogram. The older script was repository-specific, so it only took the callsign.

Test Plan: {F166042}

Reviewers: putnam, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9512