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.

Inspecione a seção idToken do README e seu exemplo de construtor. Verifique a variável referenciada pelo console log em relação à variável de token usada no exemplo e, em seguida, copie e execute o exemplo para confirmar que ele não produz mais um undefined-variable error.

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