feat(material-experimental/mdc-table): fixed section closing in sticky complex flex example

Author: forsti0506

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

@@ -75,4 +75,4 @@
     <mat-footer-row *matFooterRowDef="displayedColumns; sticky: isSticky(stickyFooters, 'footer-1')"></mat-footer-row>
     <mat-footer-row *matFooterRowDef="displayedColumns; sticky: isSticky(stickyFooters, 'footer-2')"></mat-footer-row>
   </mat-table>
-</div>
+</section>