Update 'git remote update' usage string to match man page.

Commit b344e16 taught 'git remote update' to understand
[group | remote] as its argument.  The man page was updated
to document this change, but the usage string was not.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -15,7 +15,7 @@ static const char * const builtin_remote_usage[] = {
 	"git remote set-head <name> [-a | -d | <branch>]",
 	"git remote show [-n] <name>",
 	"git remote prune [-n | --dry-run] <name>",
-	"git remote [-v | --verbose] update [-p | --prune] [group]",
+	"git remote [-v | --verbose] update [-p | --prune] [group | remote]",
 	NULL
 };