docs(auth): Updating getting started with @firebase/auth (#1634)

Author: jamesdaniels

docs/auth/getting-started.md

@@ -11,7 +11,7 @@ the Firebase docs for more information on what methods are available.](https://f
 ```ts
 import { Component } from '@angular/core';
 import { AngularFireAuth } from 'angularfire2/auth';
-import * as firebase from 'firebase/app';
+import { firebase } from '@firebase/app';
 
 @Component({
   selector: 'app-root',