docs: fix invalid headings (#24000)

PR Close #24000

packages/core/testing/src/fake_async.ts

@@ -33,7 +33,8 @@ export function resetFakeAsyncZone(): void {
  *
  * Can be used to wrap inject() calls.
  *
- * ## Example
+ * @usageNotes
+ * ### Example
  *
  * {@example core/testing/ts/fake_async.ts region='basic'}
  *
@@ -56,7 +57,8 @@ export function fakeAsync(fn: Function): (...args: any[]) => any {
  * The microtasks queue is drained at the very start of this function and after any timer callback
  * has been executed.
  *
- * ## Example
+ * @usageNotes
+ * ### Example
  *
  * {@example core/testing/ts/fake_async.ts region='basic'}
  *

packages/core/testing/src/fake_async_fallback.ts

@@ -43,7 +43,8 @@ let _inFakeAsyncCall = false;
  *
  * Can be used to wrap inject() calls.
  *
- * ## Example
+ * @usageNotes
+ * ### Example
  *
  * {@example core/testing/ts/fake_async.ts region='basic'}
  *
@@ -110,7 +111,8 @@ function _getFakeAsyncZoneSpec(): any {
  * The microtasks queue is drained at the very start of this function and after any timer callback
  * has been executed.
  *
- * ## Example
+ * @usageNotes
+ * ### Example
  *
  * {@example core/testing/ts/fake_async.ts region='basic'}
  *