"2. Documents in AngularFirestore" needs an `undefined`

In

https://github.com/angular/angularfire/blob/master/docs/firestore/documents.md

line 17 is:

```ts
item: Observable;
```

should be

```ts
item: Observable;
```

Everything else works perfectly. Thanks!

### Version info

**Angular:**

**Firebase:**

**AngularFire:**

**Other (e.g. Ionic/Cordova, Node, browser, operating system):**

### How to reproduce these conditions

**Failing test unit, Stackblitz demonstrating the problem**

**Steps to set up and reproduce**

**Sample data and security rules**

### Debug output

** Errors in the JavaScript console **

** Output from `firebase.database().enableLogging(true);` **

** Screenshots **

### Expected behavior

### Actual behavior

開啟 docs/firestore/documents.md 並檢查第 17 行，其中 AngularFirestore 文件範例宣告了 item。更新 Observable 型別以包含 undefined，然後確認該範例符合要求的型別，並且仍是有效的 TypeScript。

由索引模型根據 Issue 內容生成。