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.

Inspect the README's idToken section and its constructor example. Check the console log's referenced variable against the token variable used in the example, then copy and run the example to confirm it no longer produces an undefined-variable error.

Written by the indexing model from the issue text.