linux-phytec-ti

[media] dev-sliced-vbi.rst: use a footnote for VBI images

Just like on dvb-raw-vbi.rst, the LaTeX output doesn't work
well with cell spans. Also, this is actually a note, so, move
it to a footnote.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>