zmq: use testnet4 in zmq_sub.py example

contrib/zmq/zmq_sub.py

@@ -7,7 +7,7 @@
     ZMQ example using python3's asyncio
 
     Bitcoin should be started with the command line arguments:
-        bitcoind -testnet -daemon \
+        bitcoind -testnet4 -daemon \
                 -zmqpubrawtx=tcp://127.0.0.1:28332 \
                 -zmqpubrawblock=tcp://127.0.0.1:28332 \
                 -zmqpubhashtx=tcp://127.0.0.1:28332 \