Fix fetch example (#225)

* added anonymous function to onClick call for fetchGreeting and passed in the url prop as an argument * revised so fetchGreeting does not need the url prop passed as an arg

Author: Agentkma

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

@@ -122,7 +122,7 @@ export default function Fetch({url}) {
  const {greeting, setGreeting} = useState('')
  const {buttonClicked, setButtonClicked} = useState(false)
 
- const fetchGreeting = async url => {
+ const fetchGreeting = async () => {
  const data = await fetch(url)
  const {greeting} = data
  setGreeting(greeting)