modify magic string to constant.

spring-cloud-alibaba-docs/src/main/asciidoc-zh/rocketmq.adoc

@@ -166,7 +166,7 @@ NOTE: 在使用 RocketMQ Binder 的同时也可以配置 rocketmq.** 用于触
 MessageBuilder builder = MessageBuilder.withPayload(msg)
     .setHeader(RocketMQHeaders.TAGS, "binder")
     .setHeader(RocketMQHeaders.KEYS, "my-key")
-    .setHeader("DELAY", "1");
+    .setHeader(MessageConst.PROPERTY_DELAY_TIME_LEVEL, "1");
 Message message = builder.build();
 output().send(message);
 ```

spring-cloud-alibaba-docs/src/main/asciidoc/rocketmq.adoc

@@ -165,7 +165,7 @@ For example, `TAGS`, `DELAY`, `TRANSACTIONAL_ARG`, `KEYS`, `WAIT_STORE_MSG_OK`,
 MessageBuilder builder = MessageBuilder.withPayload(msg)
     .setHeader(RocketMQHeaders.TAGS, "binder")
     .setHeader(RocketMQHeaders.KEYS, "my-key")
-    .setHeader("DELAY", "1");
+    .setHeader(MessageConst.PROPERTY_DELAY_TIME_LEVEL, "1");
 Message message = builder.build();
 output().send(message);
 ```