MAINTAINERS: append "/" to directory patterns

It's clearer to have patterns marked as directories.

Change the directory patterns without terminating slashes.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>