[media] pixfmt-packed-rgb.rst: adjust tables to fit in LaTeX

Adjust the tables to fit at the LaTeX and PDF outputs.

I wrote a previous patch RFC to show the big table in landscape,
but it makes harder to read on displays.

So, instead, let's use the adjustbox to shrink the size of those
long tables, as the table size can still be visible on screen,
and it is a way better to read in horizontal position and
visible if printed.

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