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 e5fe406.

docs/styles/classes.md

@@ -24,7 +24,7 @@ will be closed immediately when requested.  Thus, if you are using the
 set `closeTimeoutMS` to the length (in milliseconds) of your closing
 transition.
 
-If you specify `className`, the [default content styles](README.md) will not be
+If you specify `className`, the [default content styles](index.md) will not be
 applied.  Likewise, if you specify `overlayClassName`, the default overlay
 styles will not be applied.