Fix typo and grammar (#31916)

Summary:
> Always leave the campground cleaner than you found it.

Fixing:
* typo in _dismissed_
* make the subject agree with the verb

## Changelog

[Internal] [Fixed] - A typo in a comment

Pull Request resolved: #31916

Test Plan: Grammarly says it's better now.

Reviewed By: lunaleaps

Differential Revision: D29967403

Pulled By: yungsters

fbshipit-source-id: 6cb33328e99e3fceba5f19f4baaa9446340fbbcc

packages/rn-tester/js/examples/Modal/ModalOnShow.js

@@ -132,6 +132,6 @@ export default ({
   title: "Modal's onShow/onDismiss",
   name: 'onShow',
   description:
-    'onShow and onDismiss (iOS only) callbacks are called when modals is shown/dissmissed',
+    'onShow and onDismiss (iOS only) callbacks are called when a modal is shown/dismissed',
   render: (): React.Node => <ModalOnShowOnDismiss />,
 }: RNTesterModuleExample);