Fix commit-msg hint to actually work

The change Ie4fcd0a73bd1cee12a052f38541f83aa8821c886 incorrectly
changed the command to suggest to install the commit-msg hook --
without the additional semicolon, the environment variable doesn't
seem to get set right. The braces are a cosmetic change, but nice
nonetheless.

Change-Id: Ia0a5c8f4304fb2402da8e7ea05a736bd762f3f49