Typo in Install/Setup.md Step 4

### Version info

**Angular:**
n/a

**Firebase:**
n/a

**AngularFire:**
n/a

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

### How to reproduce these conditions
By navigating to [Install and Setup.md](https://github.com/angular/angularfire/blob/HEAD/docs/install-and-setup.md) and following Step 4.

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

**Steps to set up and reproduce**
Follow the [Install and Setup.md](https://github.com/angular/angularfire/blob/HEAD/docs/install-and-setup.md) and try to build after Step 4.

**Sample data and security rules**
n/a

### Debug output

** Errors in the JavaScript console **
n/a
** Output from `firebase.database().enableLogging(true);` **
n/a
** Screenshots **

### Expected behavior
Should build/serve without any issues.

### Actual behavior
Throws error:
```
Property 'items' does not exist on type 'AppComponent'. Did you mean 'items$'?ngtsc(2551)
app.component.ts(7, 33): Error occurs in the template of component AppComponent.
```

開啟 docs/install-and-setup.md 並檢查 Step 4，然後按照連結的設定說明重現回報的建置錯誤。比較範例的範本繫結與錯誤中顯示的 AppComponent 屬性；當文件中所述的設定能夠建置並提供服務，且不再出現回報的屬性錯誤時，即視為完成。

由索引模型根據 Issue 內容生成。