fix: RadioGroup controlled example (#7763)

Author: snowystinger

packages/@react-spectrum/radio/docs/RadioGroup.mdx

@@ -133,7 +133,7 @@ The example below uses `onChange` to log how the user is interacting with the co
 
 ```tsx example
 function Example() {
-  let [selected, setSelected] = React.useState('');
+  let [selected, setSelected] = React.useState(null);
 
   return (
     <>