Change the following mongoose line according to the new standard

https://github.com/jonasschmedtmann/complete-node-bootcamp/blob/d39cbf7ee6d00538ab6ff087a5b30d00ae5409ba/4-natours/after-section-11/models/reviewModel.js#L91

```js
this.r = await this.clone().findOne()
```

- Reference - https://mongoosejs.com/docs/migrating_to_6.html#duplicate-query-execution 