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