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.

检查 README 的 idToken 部分及其构造函数示例。将 console log 引用的变量与示例中使用的 token 变量进行对照，然后复制并运行该示例，以确认它不再产生 undefined-variable error。

由索引模型根据 Issue 内容生成。