testing_hooks_util.ts: tiny comment typo fix

Author: dconeybe

packages/firestore/test/integration/util/testing_hooks_util.ts

@@ -112,7 +112,7 @@ function createExistenceFilterMismatchInfoFrom(
  * @return a promise that is fulfilled when the asynchronous work started by
  * `enablePersistentCacheIndexAutoCreation()` or
  * `disablePersistentCacheIndexAutoCreation()` completes successfully, or is
- * rejected
+ * rejected if it fails.
  */
 export function verifyPersistentCacheIndexAutoCreationToggleSucceedsDuring(
   callback: () => void