[RIP-9] Modify English document for Design_Store.md

"TransactionalMessageUtil" need not be changed to "TransactionMessageUtil ",modify the last submitted mistake.

docs/en/Design_Trancation.md

@@ -36,7 +36,7 @@ The compensation phase is used to resolve the timeout or failure case of the mes
 
 3. How Op message stored and the correspondence between Op message and Half message  
   
-  RocketMQ writes the Op message to a specific system topic(RMQ_SYS_TRANS_OP_HALF_TOPIC) which will be created via the method - TransactionMessageUtil.buildOpTopic(); this topic is an internal Topic (like the topic of RMQ_SYS_TRANS_HALF_TOPIC) and will not be consumed by the user. The content of the Op message is the physical offset of the corresponding Half message. Through the Op message we can index to the Half message for subsequent check-back operation.
+  RocketMQ writes the Op message to a specific system topic(RMQ_SYS_TRANS_OP_HALF_TOPIC) which will be created via the method - TransactionalMessageUtil.buildOpTopic(); this topic is an internal Topic (like the topic of RMQ_SYS_TRANS_HALF_TOPIC) and will not be consumed by the user. The content of the Op message is the physical offset of the corresponding Half message. Through the Op message we can index to the Half message for subsequent check-back operation.
 
 ![](../cn/image/rocketmq_design_12.png)