media: docs: cx18-streams.c: fix broken references to docs

There are two places inside this file that points to the
cx2341x documentation, with was split into two. Looking
at changeset dcc0ef88
("V4L/DVB (10442): cx18: Fixes for enforcing when Encoder Raw VBI params can be set")
with added those comments, it was originally pointing to:

	Documentation/video4linux/cx2341x/fw-encoder-api.txt

Well, the firmware details went to

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>