[media] V4L2 spec fix

Two small docbook fixes:

- prepare-buf was not positioned in alphabetical order, moved to the right place.
- the format field in create_bufs had the wrong type in the documentation

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>