[media] media: DocBook: Fix trivial typo in Sub-device Interface

When satisfied with the try results, applications can set the active formats
by setting the which argument to V4L2_SUBDEV_FORMAT_ACTIVE
not V4L2_SUBDEV_FORMAT_TRY.

Signed-off-by: Jonghun Han <jonghun.han@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>