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.

README.rst

@@ -48,7 +48,7 @@ as requested in their associated documentation.
 * `emcee <https://github.com/dfm/emcee>`__
 * `ptemcee <https://github.com/willvousden/ptemcee>`__
 * `ptmcmcsampler <https://github.com/jellis18/PTMCMCSampler>`__
-* `pypolychord <https://github.com/vhaasteren/pypolychord>`__
+* `pypolychord <https://github.com/PolyChord/PolyChordLite>`__
 * `PyMC3 <https://github.com/pymc-devs/pymc3>`_