Using the current code example, I am getting an error message Typescript error message like: 

```
Type 'Observable<(DocumentData | (DocumentData & { .... }))[]>' is not assignable to type 'Observable<Item[]>'.
```

This change also makes this code example more consistent with the current example in [firestore.md](https://github.com/angular/angularfire/blob/5793d6f3a15aa61660af75ab361c534cde9b73df/docs/firestore.md?plain=1#L91)