fix copy-paste error

Author: RytisLT

src/demo-app/tree/checklist-tree-demo/checklist-tree-demo.html

@@ -7,7 +7,7 @@
   </mat-tree-node>
   <mat-tree-node *matTreeNodeDef="let node; when: hasChild" matTreeNodePadding>
     <button mat-icon-button matTreeNodeToggle
-            [attr.aria-label]="'toggle ' + node.filename">
+            [attr.aria-label]="'toggle ' + node.item">
       <mat-icon>
         {{treeControl.isExpanded(node) ? 'expand_more' : 'chevron_right'}}
       </mat-icon>