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.

Inspecciona la sección idToken de README y su ejemplo de constructor. Comprueba la variable referenciada por console log frente a la variable de token utilizada en el ejemplo; después, copia y ejecuta el ejemplo para confirmar que ya no produce un undefined-variable error.

Escrito por el modelo de indexación a partir del texto del issue.