docs(afs): Use correct variable to construct item (#1201)

Author: xeeeveee

docs/firestore/collections.md

@@ -116,7 +116,7 @@ export class AppComponent {
  addItem(name: string) {
  // Persist a document id
  const id = this.afs.createId();
- const item: Item = { id, item };
+ const item: Item = { id, name };
  this.itemsCollection.add(item);
  }
 }