Update README.zh.md

[LinkinStars]

直接使用 README.zh.md 中的案例会报错

{
  Code: 230001,
  Msg: "Your request contains an invalid request parameter, ext=invalid message content",
  Err: {

  }
}

原因是因为消息内容 json 中的双引号没有转义的问题，所以修改 README.zh.md 添加 \ 转义

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