docs(cdk/accordion): define string type for cdkaccordion property id (#24640)

Fixes a bug in accordion/api documentation in CdkAccordion where id property is not loosley defined.
This is because id property is not explicitly defined as string.

Fixes #24639

(cherry picked from commit 853d2cc)

Author: seth0027

src/cdk/accordion/accordion.ts

@@ -36,7 +36,7 @@ export class CdkAccordion implements OnDestroy, OnChanges {
   readonly _openCloseAllActions: Subject<boolean> = new Subject<boolean>();
 
   /** A readonly id value to use for unique selection coordination. */
-  readonly id = `cdk-accordion-${nextId++}`;
+  readonly id: string = `cdk-accordion-${nextId++}`;
 
   /** Whether the accordion should allow multiple expanded accordion items simultaneously. */
   @Input()