[media] DocBook/media: fix bad spacing in VIDIOC_EXPBUF

The VIDIOC_EXPBUF documentation had spurious spaces that made it irritating
to read. Fix this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>