Merge pull request #3033 from zyh-future/zyh_asuncproducer_doc_fix

Looking at the Chinese document, I found that the subtraction operati…

docs/cn/RocketMQ_Example.md

@@ -120,11 +120,13 @@ public class AsyncProducer {
                 producer.send(msg, new SendCallback() {
                     @Override
                     public void onSuccess(SendResult sendResult) {
+                        countDownLatch.countDown();
                         System.out.printf("%-10d OK %s %n", index,
                             sendResult.getMsgId());
                     }
                     @Override
                     public void onException(Throwable e) {
+                        countDownLatch.countDown();
       	                System.out.printf("%-10d Exception %s %n", index, e);
       	                e.printStackTrace();
                     }