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/opera1.c

@@ -7,7 +7,7 @@
 *	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
 */
 
 #define DVB_USB_LOG_PREFIX "opera"