Update shorturl-en.md, Fix spell error

transofrm -> transform
Add remind about protoc, etcd-server

doc/shorturl-en.md

@@ -229,8 +229,8 @@ And now, let’s walk through the complete flow of quickly creating a microservi
       rpc shorten(shortenReq) returns(shortenResp);
   }
   ```
-
-* use goctl to generate the rpc code, execute the following command in `rpc/transofrm`
+* Confirm Protocol Buffer (protoc) whether have been installed  
+* use goctl to generate the rpc code, execute the following command in `rpc/transform`
 
   ```shell
   goctl rpc protoc transform.proto --go_out=. --go-grpc_out=. --zrpc_out=.
@@ -260,8 +260,8 @@ And now, let’s walk through the complete flow of quickly creating a microservi
   └── transformer
       └── transformer.go  
   ```
-
-  just run it, looks like:
+* Confirm etcd-server is running
+* just run it, looks like:
 
   ```shell
   $ go run transform.go -f etc/transform.yaml