get_maintainer: fix usage comment

With the `s' it just won't work.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>