docs: Fix typo in waitForAsync example (#55407)

The previous example had an incomplete code snippet that would not work if copied and pasted as is. There were two missing closing parentheses, one for the `inject` function, and the other for the `waitForAsync` function.

PR Close #55407

packages/core/testing/src/async.ts

@@ -17,7 +17,7 @@
  *   object.doSomething.then(() => {
  *     expect(...);
  *   })
- * });
+ * })));
  * ```
  *
  * @publicApi