fix #833 by changing stale link from `README.md`
fix #833 by changing stale link from `README.md`

All this does is change the "default content styles" hyperlink to show `index.md` instead of `README.md`, since I think the link to `README.md` is stale since at least [Dec 9, 2019](https://github.com/reactjs/react-modal/tree/e5fe406111f97269b8a552e9fc5ee285e05941fa/docs/styles) with commit e5fe406111f97269b8a552e9fc5ee285e05941fa.
diasbruno/react-modal,diasbruno/react-modal,reactjs/react-modal,reactjs/react-modal,diasbruno/react-modal,reactjs/react-modal