docs: say "commits" in the --depth option wording for git-clone

It is not wrong to talk about "revisions" here, but in this context
revisions are always commits, and that is how we already name it in the
git-fetch docs. So align the docs by always referring to "commits".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -178,7 +178,7 @@ objects from the source repository into a pack in the cloned repository.
 
 --depth <depth>::
 	Create a 'shallow' clone with a history truncated to the
-	specified number of revisions. Implies `--single-branch` unless
+	specified number of commits. Implies `--single-branch` unless
 	`--no-single-branch` is given to fetch the histories near the
 	tips of all branches.