maintainers.inc: rename gtk-doc-stub to gtk-doc, reassign to me

(From meta-yocto rev: e09444f0f613cd7b092bab5cb0106c1447be1ecf)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>