Fix comment in spec tests (#4992)

Author: schmidt-sebastian

packages/firestore/test/util/test_platform.ts

@@ -60,7 +60,7 @@ export class FakeWindow implements WindowLike {
         break;
       case 'pagehide':
       case 'visibilitychange':
-        // The spec tests currently do not rely on `unload`/`visibilitychange`
+        // The spec tests currently do not rely on `pagehide`/`visibilitychange`
         // listeners.
         break;
       default: