Fix link in README to the correct version of PyPolyChord
The current link to pypolychord in the README file does not point to the version of PyPolyChord that is actually implemented in bilby. This MR fixes the link to the correct github repo.
The current link to pypolychord in the README file does not point to the version of PyPolyChord that is actually implemented in bilby. This MR fixes the link to the correct github repo.