[media] docs-rst: Fix some typos

Those fixes came from patchs from Andrea for the old DocBook
documentation.

As we're removing it on Kernel 4.8, it doesn't make sense to
apply the original patches, but, as the typos were ported
to ReST, let's fix the issues there.

Suggested-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>