docs: add missing useState import to React Transition Group example (testing-library#1080)

Author: mrazauskas

docs/example-react-transition-group.mdx

@@ -54,7 +54,7 @@ test('you can mock things with jest.mock', () => {
 ## Shallow
 
 ```jsx
-import React from 'react'
+import React, {useState} from 'react'
 import {CSSTransition} from 'react-transition-group'
 import {render, fireEvent} from '@testing-library/react'