Documentation example doesn't work out of the box: metadata provider

The example of getting code positions using ``PositionProvider`` no longer works, as the API has changed from ``libcst.PositionProvider`` to ``libcst.metadata.PositionProvider``.

Example at https://github.com/Instagram/LibCST/blob/master/docs/source/metadata.rst#metadata-apis.

Fixing this is obviously trivial, but was wondering if you'd considered setting up doctests to prevent this kind of mistake slipping in?