cmd_git_gc: fix typo in usage string
cmd_git_gc: fix typo in usage string

Suggested by oconnor663.

Issue: #9381

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client