MAINTAINERS: update location of media main tree

There were some recent changes on the way we're handling
media patches. Now, the official tree is located at:

	https://git.linuxtv.org/media.git/



Update it at MAINTAINERS file.

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