README: update @testing-library/jest-dom's old syntax (#590)

[@testing-library/jest-dom v5.0.0](https://github.com/testing-library/jest-dom/releases/tag/v5.0.0) later, import statement had BreakingChange.
So update to latest that in the README example.

Thank you 😀

Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>

Author: ryota-murakami

README.md

@@ -171,7 +171,7 @@ export default HiddenMessage
 // __tests__/hidden-message.js
 // these imports are something you'd normally configure Jest to import for you
 // automatically. Learn more in the setup docs: https://testing-library.com/docs/react-testing-library/setup#cleanup
-import '@testing-library/jest-dom/extend-expect'
+import '@testing-library/jest-dom'
 // NOTE: jest-dom adds handy assertions to Jest and is recommended, but not required
 
 import React from 'react'
@@ -261,7 +261,7 @@ export default Login
 // __tests__/login.js
 // again, these first two imports are something you'd normally handle in
 // your testing framework configuration rather than importing them in every file.
-import '@testing-library/jest-dom/extend-expect'
+import '@testing-library/jest-dom'
 import React from 'react'
 import {render, fireEvent, screen} from '@testing-library/react'
 import Login from '../login'