docs: in react hooks example, limit createEmpty calls (facebookarchive#2432)

Summary:
In the React hooks example, `EditorState.createEmpty` will be called each time the component is rendered. By wrapping it in a function, it will only be called once per instance of the MyEditor component.
Pull Request resolved: facebookarchive#2432

Reviewed By: claudiopro

Differential Revision: D21652207

Pulled By: mrkev

fbshipit-source-id: 0c24a4e84718227b4e89d1c810b2558ffd51384d

Author: benatkin

docs/Overview.md

@@ -75,7 +75,7 @@ import 'draft-js/dist/Draft.css';
 
 function MyEditor() {
   const [editorState, setEditorState] = React.useState(
-    EditorState.createEmpty(),
+    () => EditorState.createEmpty(),
   );
 
   return <Editor editorState={editorState} onChange={setEditorState} />;