tidy up example

apps/docs/content/components/FormControl.mdx

@@ -236,7 +236,7 @@ Try changing the input value to `monalisa` and submitting the form to show the `
 
 ```javascript live noinline
 const App = () => {
-  const usernameInputRef = React.useRef(null)
+  const inputRef = React.useRef(null)
   const [value, setValue] = React.useState('mona lisa')
   const [isValid, setIsValid] = React.useState(false)
 
@@ -248,7 +248,7 @@ const App = () => {
     setIsValid(valid)
 
     if (!valid) {
-      usernameInputRef.current.focus()
+      inputRef.current.focus()
     }
   }
 
@@ -258,8 +258,7 @@ const App = () => {
         <FormControl validationStatus={isValid ? 'success' : 'error'} fullWidth>
           <FormControl.Label>GitHub handle</FormControl.Label>
           <TextInput
-            name="username"
-            ref={usernameInputRef}
+            ref={inputRef}
             fullWidth
             value={value}
             onChange={onChange}