`magit-key-mode-groups': 'committing': fix parens

Commit b0954e50 moved the "--gpg-sign" flag from 'arguments'
to 'switches', but didn't properly adjust the parentheses.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #776 from praet/paren-fix

`magit-key-mode-groups': 'committing': fix parens