Cast collectionData to avoid TS errors (#3434)

* Cast collectionData to avoid TS errors

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)

* accept reviewer's suggestion

---------

Co-authored-by: James Daniels <[email protected]>

Author: ahmetkizilay

README.md

@@ -58,7 +58,7 @@ export class AppComponent {
 
   constructor() {
     const itemCollection = collection(this.firestore, 'items');
-    this.item$ = collectionData(itemCollection) as Observable<Item[]>;
+    this.item$ = collectionData<Item>(itemCollection);
   }
 }
 ```