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>

Documentation/userspace-api/media/v4l/dev-sliced-vbi.rst

@@ -127,7 +127,7 @@ struct v4l2_sliced_vbi_format
     :stub-columns: 0
     :widths:       3 3 2 2 2
 
-    * - __u32
+    * - __u16
       - ``service_set``
       - :cspan:`2`