feat(material-experimental/mdc-table): fixed table sticky complex flex example accessibility

Author: forsti0506

src/components-examples/material-experimental/mdc-table/table-sticky-complex-flex/table-sticky-complex-flex-example.html

@@ -35,7 +35,7 @@
   </mat-button-toggle-group>
 </div>
 
-<div class="example-container mat-elevation-z8">
+<section class="example-container mat-elevation-z8" tabindex="0">
   <mat-table [dataSource]="dataSource" *ngFor="let table of tables">
     <ng-container matColumnDef="position" [sticky]="isSticky(stickyColumns, 'position')">
       <mat-header-cell *matHeaderCellDef> Position </mat-header-cell>