fix(accordion): uncomment cdk accordion example, add cdkAccordionItem selector (#11131)

Author: josephperrott

src/cdk/accordion/accordion-item.ts

@@ -28,7 +28,7 @@ let nextId = 0;
  * events and attributes needed to be managed by a CdkAccordion parent.
  */
 @Directive({
-  selector: 'cdk-accordion-item',
+  selector: 'cdk-accordion-item, [cdkAccordionItem]',
   exportAs: 'cdkAccordionItem',
 })
 export class CdkAccordionItem implements OnDestroy {

src/demo-app/expansion/expansion-demo.html

@@ -74,8 +74,8 @@ <h1>matAccordion</h1>
     </mat-action-row>
   </mat-expansion-panel>
 </mat-accordion>
-<!--
-<h1>CdkAccordion</h1>
+
+<h1>cdkAccordion</h1>
 <div>
   <p>Accordion Options</p>
   <div>
@@ -108,4 +108,4 @@ <h1>CdkAccordion</h1>
     <p *ngIf="item3.expanded">I only show if item 3 is expanded</p>
   </cdk-accordion-item>
 </cdk-accordion>
- -->
+