MAINTAINERS: ieee802154: update documentation file pattern

When moving the documentation for the ieee802154 subsystem from
plain text to rst the file pattern in the MAINTAINERS file got wrong.
Updating it here to fix scripts using this file.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>