Documentation/fb/viafb.modes fix a typo

Corrected the word negtive to negative.

Signed-off-by: Stefan Huber <steffhip@gmail.com>
Signed-off-by: Matthias Schid <aircrach115@gmail.com>
Signed-off-by: Simon Puels <simon.puels@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>