[media] Documentation/DocBook/media/v4l: fix typo, s/packet/packed/

Change "packet" to "packed" where the doc is talking about packed data
formats.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>