build: fix drag-drop demo

I left in some invalid code in the `drag-drop` demo by accident while working on #20668 which is throwing an error now.

Author: crisbeto

src/dev-app/drag-drop/drag-drop-demo.html

@@ -9,7 +9,7 @@ <h2>To do</h2>
       [cdkDropListData]="todo">
       <div *ngFor="let item of todo" cdkDrag>
         {{item}}
-        <ng-container cdkDragHandle>move</ng-container>
+        <mat-icon cdkDragHandle svgIcon="dnd-move"></mat-icon>
       </div>
     </div>
   </div>