README.md misused variable

Head to https://github.com/angular/angularfire/blob/master/README.md

### 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
**Failing test unit, Stackblitz demonstrating the problem**
N/A
**Steps to set up and reproduce**
N/A
**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**
N/A
### Related Issues
[#3343](https://github.com/angular/angularfire/issues/3343)
### Expected behavior
The declared retrieved collection `itemCollection` is to be used by passing it into the `collectionData` method as a parameter.
``` typescript
constructor() {
const itemCollection = collection(this.firestore, 'items');
this.item$ = collectionData(itemCollection);
}
```
### Actual behavior
The declared retrieved collection `itemCollection` is not being used, instead an unexisting collection `collection` is being used.
``` typescript
constructor() {
const itemCollection = collection(this.firestore, 'items');
this.item$ = collectionData(collection);
}
```

打开 README.md，检查 issue 中链接的 TypeScript 示例。将声明的 itemCollection 与传递给 collectionData 的参数进行比较，然后更新示例，使其使用已声明的 collection，并确认显示的代码片段保持一致。

由索引模型根据 Issue 内容生成。