fix(demo): The typeaheadAsync property was missing, which I found to …

…be necessary for the dropdown/functionality to be used. (#4662)

demo/src/app/components/+typeahead/demos/async/async.html

@@ -1,6 +1,7 @@
 <pre class="card card-block card-header">Model: {{asyncSelected | json}}</pre>
 
 <input [(ngModel)]="asyncSelected"
+       [typeaheadAsync]="true"
        [typeahead]="dataSource"
        (typeaheadLoading)="changeTypeaheadLoading($event)"
        (typeaheadOnSelect)="typeaheadOnSelect($event)"