Fix typo in config.go

config.go

@@ -1508,7 +1508,7 @@ func extractBitcoindRPCParams(bitcoindConfigPath string) (string, string, string
 // ZMQ rawblock and rawtx notifications are different.
 func checkZMQOptions(zmqBlockHost, zmqTxHost string) error {
 	if zmqBlockHost == zmqTxHost {
-		return errors.New("zmqpubrawblock and zmqpubrawtx must be set" +
+		return errors.New("zmqpubrawblock and zmqpubrawtx must be set " +
 			"to different addresses")
 	}