scripts/checklayer/common.py: Fixed a minor grammatical error

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8ea17456)
Signed-off-by: Steve Sakoman <steve@sakoman.com>