Documentation: fb, fix path to modedb.c

modedb.c was moved twice since the modedb documentation was written.
Update the path to the current one.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200723103454.26167-1-jslaby@suse.cz


Signed-off-by: Jonathan Corbet <corbet@lwn.net>