Correct the comments in JS SDK example

File: [/zh_CN/development/javascript.md](https://docs.emqx.cn/cn/broker/latest/development/javascript.html)

```
// const mqtt = require('mqtt')
import mqtt from 'mqtt'

// 连接选项
const options = {
clean: true, // 保留回话
connectTimeout: 4000, // 超时时间
// 认证信息
clientId: 'emqx_test',
username: 'emqx_test',
password: 'emqx_test',
}
```

The comments after `clean: true` should be "true: 清除会话, false: 保留会话".