file rename

doc/design/cluster_train/master_process.md → doc/design/cluster_train/master_server.md

 # Design Doc: Master Server
 
-For an overview of master server' role, please refer to [distributed training design doc](./README.md). In this design doc we will discuss the master server in more details. The master will be implemented in [Go](https://golang.org/).
+For an overview of master server's role, please refer to [distributed training design doc](./README.md). In this design doc we will discuss the master server in more details. The master will be implemented in [Go](https://golang.org/).
 
 ## Dataset