perfect the bookstore and shorturl doc (#109)

* perfect the bookstore and shorturl doc

* 避免歧义

doc/shorturl-en.md

@@ -72,7 +72,7 @@ And now, let’s walk through the complete flow of quickly create a microservice
   GO111MODULE=on go get -u github.com/tal-tech/go-zero/tools/goctl
   ```
 
-* create the working dir `shorturl`
+* create the working dir `shorturl` and `shorturl/api`
 
 * in `shorturl` dir, execute `go mod init shorturl` to initialize `go.mod`
 
@@ -195,6 +195,8 @@ And now, let’s walk through the complete flow of quickly create a microservice
 
 ## 6. Write code for transform rpc service
 
+- under directory `shorturl` create dir `rpc`
+
 * under directory `rpc/transform` create `transform.proto` file
 
   ```shell