[Doc] Operations_Consumer.md backlog snippet uses non-existent Message.PROPERTY_MAX_OFFSET

Description

The consumer-accumulation monitoring snippet in docs/en/Operations_Consumer.md (section on consumption backlog) references a constant on the wrong class:

String maxOffset =
            msgs.get(0).getProperty(Message.PROPERTY_MAX_OFFSET);

Message has no PROPERTY_MAX_OFFSET constant — it is defined in org.apache.rocketmq.common.message.MessageConst (MessageConst.PROPERTY_MAX_OFFSET, common/src/main/java/org/apache/rocketmq/common/message/MessageConst.java:33). The snippet as printed does not compile (cannot find symbol: variable PROPERTY_MAX_OFFSET).

Expected behavior

The snippet should read msgs.get(0).getProperty(MessageConst.PROPERTY_MAX_OFFSET) with MessageConst imported (or fully qualified).