docs(auth): log the token value in the idToken example (#3728)

The idToken example's subscription logged `string`, a type name rather
than a value in that scope, so the snippet would not compile if copied.
Log the `token` parameter the callback receives, matching the neighboring
authState example. Also correct the comment on the line above, copied
from the authState example, which referred to a user rather than the
token this example streams.

Reland of #3652 by saadlalam, whose fix was correct but could not be
merged because the CLA check never cleared on that branch.

Author: armando-navarro

docs/auth.md

@@ -171,8 +171,8 @@ export class UserComponent implements OnDestroy {
 
   constructor() {
     this.idTokenSubscription = this.idToken$.subscribe((token: string | null) => {
-        //handle idToken changes here. Note, that user will be null if there is no currently logged in user.
-     console.log(string);
+        //handle idToken changes here. Note, that token will be null if there is no currently logged in user.
+     console.log(token);
     })
   }