update step 57

Author: ziaukhan

step57_firebase_list/app/app.component.ts

@@ -17,7 +17,7 @@ import { AngularFire, FirebaseListObservable } from 'angularfire2';
  `,
 })
 export class AppComponent {
- items: FirebaseListObservable<any>;
+ items: FirebaseListObservable<any[]>;
  constructor(af: AngularFire) {
  this.items = af.database.list('/messages');
  }