checkkconfigsymbols.py: Fix typo in help message

Fix a typo in the help message for the -d parameter by removing one 'm'.

Signed-off-by: Andreas Ziegler <andreas.ziegler@fau.de>
Acked-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>