git-arc: Properly quote .nm arg

.Nm git arc has 2 args, despite the mdoc reference saying there's only
one arg when in the preamble and NAME sections, but not noting that
restriction later. Quote the arg and bump the date. This doesn't affect
man git-arc, but does affect the rendering of the man page on at least
one website. Date bumped since it is a semantic change and not just
a typo or other trivial correction.

Reviewed by: jhb
Sponsored by: Netflix