Use jsx instead of js for the code example.

docs/reference-guides/slotfills/plugin-sidebar.md

@@ -5,7 +5,7 @@ Using this slot will add an icon to the toolbar that, when clicked, opens a pane
 
 ## Example
 
-```js
+```jsx
 import { __ } from '@wordpress/i18n';
 import { PluginSidebar } from '@wordpress/edit-post';
 import {