Fix the syntax for an example in user-authentication docs. (#422)

* Fix the syntax to fetch the correct authencation response.
* With the current syntax an exception is thrown: "cannot read property null of uid"

Author: aashil

docs/5-user-authentication.md

@@ -87,7 +87,7 @@ import { AngularFire } from 'angularfire2';
   moduleId: module.id,
   selector: 'app',
   template: `
-  <div> {{ (af.auth | async).uid }} </div>
+  <div> {{ (af.auth | async)?.uid }} </div>
   <button (click)="login()">Login</button>
   `,
 })