V4L/DVB (10134): v4l2 doc: set v4l2_dev instead of parent.

Update the documentation now that the v4l2_dev field is in.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>