media: dvb: point to the location of the old README.dvb-usb file

This file got renamed, but the references still point to the
old place.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>

drivers/media/usb/dvb-usb/m920x.c

@@ -6,7 +6,7 @@
  *	under the terms of the GNU General Public License as published by the
  *	Free Software Foundation, version 2.
  *
- * see Documentation/dvb/README.dvb-usb for more information
+ * see Documentation/media/dvb-drivers/dvb-usb.rst for more information
  */
 
 #include "m920x.h"