Spelling Mistake in Readme of Auth Section

### Version info

Not version related since it's in the readme of the project.

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

### How to reproduce these conditions

Copy the constructor code of the README file and run it anywhere in your project.

In the constructor of README the console log prints a variable that is not declared and produces undefined variable error.

**Sample data and security rules**

### Debug output

** Screenshots **

### Expected behavior

In the constructor of the section idToken in the README file the console log should be console.log(token);

### Actual behavior

In the constructor of the section idToken in the README file the console log is trying to print a variable called string that does not exist.

檢查 README 的 idToken 區段及其建構函式範例。將 console log 參照的變數與範例中使用的 token 變數進行比對，然後複製並執行該範例，以確認它不再產生 undefined-variable error。

由索引模型根據 Issue 內容生成。