[media] dev-sliced-vbi.rst: fix reference for v4l2_mpeg_vbi_ITV0

The struct v4l2_mpeg_vbi_ITV0 is identical to struct v4l2_mpeg_vbi_itv0,
except by its size, and it is documented at the same place at the
book.

Fix cross reference for it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/dev-sliced-vbi.rst

@@ -672,8 +672,10 @@ Magic Constants for struct v4l2_mpeg_vbi_fmt_ivtv magic field
 
 .. c:type:: v4l2_mpeg_vbi_itv0
 
-struct v4l2_mpeg_vbi_itv0
--------------------------
+.. c:type:: v4l2_mpeg_vbi_ITV0
+
+structs v4l2_mpeg_vbi_itv0 and v4l2_mpeg_vbi_ITV0
+-------------------------------------------------
 
 .. tabularcolumns:: |p{4.4cm}|p{2.4cm}|p{10.7cm}|