Fix grammatical error in git-revert

Fix grammatical error in git-revert

We always talk about "commit xyz".

We never talk about "xyz commit", except when we end up talking
about a commit as a branch head (notably, I would say "the HEAD
commit", or possibly "the top-of-master commit", but here
$commit is a SHA1 name, not anything else).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>