video: matroxfb: Correct video option in comments and kernel config help.

Configuring the kernel I found that the Matrox frame buffer help has a
different option than the one in the docs (Documentation/fb/matroxfb.txt).
I decided to check the source code to see what is the correct option.
drivers/video/matrox/matroxfb_base.c has a lot of comments that sugests
that the video option is "matrox".
However in line 2452 of this same file you have:
fb_get_options("matroxfb", &option)

video=matroxfb:XXX is the correct video option not video=matrox:XXX.

Signed-off-by: Vicente Jimenez Aguilar <googuy@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>