media: vidioc-g-tuner.rst: Fix table number of cols

The Tuner Audio Matrix table is broken: the first row has 7
columns instead of 6, causing it to be parsed wrong and displayed
very badly on PDF output.

Fix it and adjust the table to look nice at PDF output

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