[media] DocBook: fix an unbalanced <para> tag

The </para> got lost on some change at the DVB docbook, making
the tag unbalanced and causing a warning. Fix it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>