docs: fix typo in example for list of items

this.items = af.database.list('/items');
previously 
this.items = af.database.list('items');

Author: Akash Paul

docs/1-install-and-setup.md

@@ -124,7 +124,7 @@ import { AngularFire, FirebaseListObservable } from 'angularfire2';
 export class AppComponent {
   items: FirebaseListObservable<any[]>;
   constructor(af: AngularFire) {
-    this.items = af.database.list('items');
+    this.items = af.database.list('/items');
   }
 }
 ```