[media] DocBook media: fix typo in V4L2_CTRL_FLAG_EXECUTE_ON_WRITE

Fix small typo (missing 'it') in the documentation for
V4L2_CTRL_FLAG_EXECUTE_ON_WRITE.

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>