Fix the doc badge link
.. image:: https://coveralls.io/repos/github/lmcinnes/umap/badge.svg
:target: https://coveralls.io/github/lmcinnes/umap
:alt: Test Coverage Status
-.. image:: //readthedocs.org/projects/umap-learn/badge/?version=latest
+.. image:: https://readthedocs.org/projects/umap-learn/badge/?version=latest
:target: https://umap-learn.readthedocs.io/en/latest/?badge=latest
:alt: Documentation Status
