[Beta] Add missing imports to ref pages (#5442)

* [Beta] Add missing imports to ref pages

* Fix spaces

beta/src/content/reference/react-dom/unmountComponentAtNode.md

@@ -31,6 +31,8 @@ unmountComponentAtNode(domNode)
 Call `unmountComponentAtNode` to remove a mounted React component from the DOM and clean up its event handlers and state.
 
 ```js
+import { unmountComponentAtNode } from 'react-dom';
+
 const domNode = document.getElementById('root');
 render(<App />, domNode);