[media] DocBook/media: add missing entry for V4L2_PIX_FMT_Y16_BE

This format was added but not documented. Do this now.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>