media: dvb-frontend-parameters.rst: fix the name of a struct

The struct that contains an union of DVB parameters is
called dvb_frontend_parameters (and not FrontendParameters).

Fix it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/dvb/dvb-frontend-parameters.rst

@@ -24,7 +24,7 @@ instead, in order to be able to support the newer System Delivery like
 DVB-S2, DVB-T2, DVB-C2, ISDB, etc.
 
 All kinds of parameters are combined as a union in the
-FrontendParameters structure:
+``dvb_frontend_parameters`` structure:
 
 
 .. code-block:: c