fix the controlled RadioGroup example (#6057)

fix the controlled RadioGroup example (#6057)

Author: sookmax

packages/@react-aria/radio/docs/useRadioGroup.mdx

@@ -268,7 +268,7 @@ The `onChange` event is fired when the user selects a radio.
 
 ```tsx example
 function Example() {
-  let [selected, setSelected] = React.useState('');
+  let [selected, setSelected] = React.useState(null);
 
   return (
     <>