fix mention of Return.Successes

this setting was renamed twice. See:
- 5411ec3676ba932f988c614650cdc314b8def468
- 4189e55a44bd4e04772171b5e7a7b6dcd12603f6

async_producer.go

@@ -33,7 +33,7 @@ type AsyncProducer interface {
 	// wish to send.
 	Input() chan<- *ProducerMessage
 
-	// Successes is the success output channel back to the user when AckSuccesses is
+	// Successes is the success output channel back to the user when Return.Successes is
 	// enabled. If Return.Successes is true, you MUST read from this channel or the
 	// Producer will deadlock. It is suggested that you send and read messages
 	// together in a single select statement.