[media] videobuf2-core.h: fix comment

The comment for start_streaming that tells the developer with which vb2 state
buffers should be returned to vb2 gave the wrong state. Very confusing.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>