media: dev-sliced-vbi.rst: fix wrong type

The documentation reported service_set as a __u32, but according to
videodev2.h it is a __u16. Correct the documentation.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>