media: frontend.rst: fix supported delivery systems

The introduction for the frontend chapter is not quite
correct:
  - it tells that it supports only three types of
    delivery systems, in opposite to three *groups*;
  - It adds ISDB-C to the list of supported systems,
    but, this is not true.

Fix it.

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