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/gp8psk.c

@@ -12,7 +12,7 @@
  *	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 "gp8psk.h"
 #include "gp8psk-fe.h"