Update README due to deprecation of react-testing-library's toBeInTheDOM (facebook#5334)

Author: rrebase

Diff for: template/README.md

@@ -1496,7 +1496,7 @@ import App from './App';
 
 it('renders welcome message', () => {
   const { getByText } = render(<App />);
-  expect(getByText('Welcome to React')).toBeInTheDOM();
+  expect(getByText('Welcome to React')).toBeInTheDocument();
 });
 ```