media: vidioc-g-fmt.rst: adjust table format

While doing a visual inspection with Sphinx 1.5, I noticed that
one of the columns was smaller than the text written there.

As this is the only thing I noticed with Sphinx 1.5, I suspect
that this was also a problem with Sphinx 1.4. Yet, I opted to
touch it in a way that wouldn't cause backward issues.

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