message: fix struct name in comment (#520)

Signed-off-by: hongmengning <[email protected]>
Co-authored-by: Miłosz Smółka <[email protected]>

message/message.go

@@ -42,7 +42,7 @@ type Message struct {
 
 	// ack is closed when acknowledge is received.
 	ack chan struct{}
-	// noACk is closed when negative acknowledge is received.
+	// noAck is closed when negative acknowledge is received.
 	noAck chan struct{}
 
 	ackMutex    sync.Mutex