Update README.md

Author: 256dpi

README.md

@@ -133,7 +133,7 @@ void onMessage(MQTTClientCallbackSimple);
 // Callback signature: void messageReceived(String &topic, String &payload) {}
 
 void onMessageAdvanced(MQTTClientCallbackAdvanced);
-// Callback signature: void messageReceived(MQTTClient *client, char[] topic, char payload[], int payload_length) {}
+// Callback signature: void messageReceived(MQTTClient *client, char topic[], char payload[], int payload_length) {}
 ```
 
 - The set callback is mostly called during a call to `loop()` but may also be called during a call to `subscribe()`, `unsubscribe()` or `publish() // QoS > 0` if messages have been received before receiving the required acknowledgement. Therefore, it is strongly recommended to not call `subscribe()`, `unsubscribe()` or `publish() // QoS > 0` directly in the callback.