MAINTAINERS: fix Documentation/mei/

The documentation was moved to Documentation/misc-devices/mei/ instead.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>