[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>