fix example

README.md

@@ -31,7 +31,7 @@ from hitbtc import HitBTC
 c = HitBTC()
 c.start()  # start the websocket connection
 time.sleep(2)  # Give the socket some time to connect
-c.subscribe_ticker('ETHBTC') # Subscribe to ticker data for the pair ETHBTC
+c.subscribe_ticker(symbol='ETHBTC') # Subscribe to ticker data for the pair ETHBTC
 
 while True:
     try: