Update intro.mdx to use named export for userEvent

As per testing-library/user-event#1205

Author: ahayes91

docs/user-event/intro.mdx

@@ -64,7 +64,7 @@ itself - e.g. in a `before`/`after` hook - for reasons described in
 ["Avoid Nesting When You're Testing"](https://kentcdodds.com/blog/avoid-nesting-when-youre-testing).
 
 ```js
-import userEvent from '@testing-library/user-event'
+import {userEvent} from '@testing-library/user-event'
 
 // inlining
 test('trigger some awesome feature when clicking the button', async () => {
@@ -80,7 +80,7 @@ test('trigger some awesome feature when clicking the button', async () => {
 ```
 
 ```js
-import userEvent from '@testing-library/user-event'
+import {userEvent} from '@testing-library/user-event'
 
 // setup function
 function setup(jsx) {