doc-rst: v4l2: Rename the V4L2 API title

The V4L2 is the only part of the doc that has the word "Specification"
and mentions its version on the title.

Having the version there was important in the past, while we were
getting rid of V4L version 1. But, as v1 is long gone, all it lasts
is history (with is, btw, covered on the spec). So, no need to keep
the version on its title.

So, rename it, to be more generic and look like the remaining
of the document.

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