"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

Open docs/firestore/documents.md and inspect line 17, where the AngularFirestore document example declares item. Update the Observable type to include undefined, then verify the example matches the requested type and remains valid TypeScript.

Written by the indexing model from the issue text.