rtpengine: add example in doc for write_sdp_pv

modules/rtpengine/doc/rtpengine_admin.xml

@@ -302,9 +302,12 @@ modparam("rtpengine", "force_send_interface", "2001:8d8:1ff:10c0:9a90:96ff:fea8:
 <programlisting format="linespecific">
 ...
 modparam("rtpengine", "write_sdp_pv", "$avp(sdp)")
-  ...  or
-modparam("rtpengine", "write_sdp_pv", "$pv(sdp)")
 ...
+route {
+	...
+	rtpproxy_manage();
+	set_body("$avp(sdp)a=baz123\r\n", "application/sdp");
+}
 </programlisting>
 		</example>
 	</section>