Update method in comment for getting the id token (#262)

Sources
- https://github.com/firebase/firebase-js-sdk/blob/master/packages/auth/src/model/user.ts#L85
- https://firebase.google.com/docs/reference/js/v8/firebase.User#getidtoken
- https://firebase.google.com/docs/reference/js/auth.user.md#usergetidtoken
- https://firebase.google.com/docs/auth/web/manage-users#get_a_users_profile

auth/manage.js

@@ -16,7 +16,7 @@ function getUserProfile() {
 
     // The user's ID, unique to the Firebase project. Do NOT use
     // this value to authenticate with your backend server, if
-    // you have one. Use User.getToken() instead.
+    // you have one. Use User.getIdToken() instead.
     const uid = user.uid;
   }
   // [END auth_get_user_profile]