V4L/DVB: v4l doc: fix font of field name

The field 'reserved' was not tagged as a 'structfield' as it should.
This made the text a bit confusing.

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