kamailio.cfg: fixed type in nat sipping flag name

- reported by Rouskol Andrey

etc/kamailio.cfg

@@ -546,7 +546,7 @@ route[REGISTRAR] {
 		{
 			setbflag(FLB_NATB);
 			# uncomment next line to do SIP NAT pinging 
-			## setbflag(FLB_SIPPING);
+			## setbflag(FLB_NATSIPPING);
 		}
 		if (!save("location"))
 			sl_reply_error();