docs: format markdown and add go mod in demo (#155)

readme-en.md

@@ -164,6 +164,8 @@ go get -u github.com/tal-tech/go-zero
 
    ```shell
    cd greet
+   go mod init
+   go mod tidy
    go run greet.go -f etc/greet-api.yaml
    ```
 

readme.md

@@ -108,6 +108,8 @@ GO111MODULE=on GOPROXY=https://goproxy.cn/,direct go get -u github.com/tal-tech/
     ```shell
     goctl api new greet
     cd greet
+    go mod init
+    go mod tidy
     go run greet.go -f etc/greet-api.yaml
     ```
 
@@ -121,8 +123,11 @@ GO111MODULE=on GOPROXY=https://goproxy.cn/,direct go get -u github.com/tal-tech/
 
     ```http
     HTTP/1.1 200 OK
-   Date: Sun, 30 Aug 2020 15:32:35 GMT
-   Content-Length: 0
+    Content-Type: application/json
+	Date: Thu, 22 Oct 2020 14:03:18 GMT
+	Content-Length: 14
+
+	{"message":""}
     ```
 
     编写业务代码：