docs: media: update location of the media patches

Due to recent changes on the way we're maintaining media, the
location of the main tree was updated.

Change docs accordingly.

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>