Documentation: fix broken v4l-utils URL

This replaces http://git.linuxtv.org/v4l-utils/ (broken link)
by http://git.linuxtv.org/cgit.cgi/v4l-utils.git/

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>