文档-模型管理和权限管理中示例代码错误
**描述 bug**

- [文档链接](http://doc.cms.7yue.pro/lin/server/koa/authority_model.html#%E6%9D%83%E9%99%90%E7%AE%A1%E7%90%86)

文档原文节选：
打开fake.js文件，换成如下代码：
```js
require("./initial");
const { db } = require("lin-mizar/lin/db");
// eslint-disable-next-line no-unused-vars
const { User, Group, Auth } = require("lin-mizar/lin");

const run = async () => {
  const group = await Group.findOne({
    where: {
      name: "普通分组"
    }
  });
  const user = await User.findOne({
    where: {
      username: "pedro"
    }
  });
  user.group_id = group.id;
  await user.save();

  db.close();
};

run();
```
-  此处代码直接运行会出错

正确示例代码应该是：
```js
require("./initial");
const { db } = require("lin-mizar/lin/db");
// eslint-disable-next-line no-unused-vars
const { User, Group, Auth } = require("lin-mizar/lin");
const { initModels } = require("./init_models")

const run = async () => {
  await initModels();
  const group = await Group.findOne({
    where: {
      name: "普通分组"
    }
  });
  const user = await User.findOne({
    where: {
      username: "pedro"
    }
  });
  user.group_id = group.id;
  await user.save();

  db.close();
};

run();
```
- （增加了 initModels 的导入和调用）

**你使用哪个版本出现该问题？**

 master 分支
