[media] doc-rst: Convert dvb-usb to ReST format

This file is not on any markup language. Convert it to
ReST format.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/dvb-drivers/index.rst

@@ -23,4 +23,5 @@ License".
 	bt8xx
 	cards
 	ci
+	dvb-usb
 	contributors