Use simplified import of @testing-library/jest-dom (facebook#8763)

Author: Dremora

docusaurus/docs/running-tests.md

@@ -194,7 +194,7 @@ Similar to `enzyme` you can create a `src/setupTests.js` file to avoid boilerpla
 ```js
 // react-testing-library renders your components to document.body,
 // this adds jest-dom's custom assertions
-import '@testing-library/jest-dom/extend-expect';
+import '@testing-library/jest-dom';
 ```
 
 Here's an example of using `react-testing-library` and `jest-dom` for testing that the `<App />` component renders "Welcome to React".