minor fixes in example-intro.md (testing-library#529)

Few minor things were missing when I was trying out.

Author: balavishnuvj

docs/react-testing-library/example-intro.md

@@ -171,7 +171,7 @@ expect(screen.getByRole('button')).not.toHaveAttribute('disabled')
 fetch.js
 
 ```jsx
-import React, { useState } from 'react'
+import React, { useState, useReducer } from 'react'
 import axios from 'axios'
 
 function greetingReducer(state, action) {
@@ -182,7 +182,7 @@ function greetingReducer(state, action) {
  greeting: action.greeting,
  }
  }
- case: 'ERROR': {
+ case 'ERROR': {
  return {
  error: action.error,
  greeting: null
@@ -207,7 +207,7 @@ export default function Fetch({ url }) {
  setButtonClicked(true)
  })
  .catch((error) => {
- dispatch({ type: 'ERROR' })
+ dispatch({ type: 'ERROR', error })
  })
  }