Fix: producer code example (apache#166)

Fixing a typo in a readme.md following changes done in apache#141

README.md

@@ -64,7 +64,7 @@ producer, err := client.CreateProducer(pulsar.ProducerOptions{
 	Topic: "my-topic",
 })
 
-err = producer.Send(context.Background(), &pulsar.ProducerMessage{
+_, err = producer.Send(context.Background(), &pulsar.ProducerMessage{
 	Payload: []byte("hello"),
 })