Update references.txt
Removed broken link to conda cheat sheet. Changed to more stable (?) link.
.. _pip: https://pip.pypa.io/en/stable/
.. _pip install instructions: https://pip.pypa.io/en/latest/installing.html#install-pip
.. _conda: https://conda.io/docs/
-.. _conda cheat sheet: https://conda.io/docs/_downloads/conda-cheatsheet.pdf
+.. _conda cheat sheet: https://docs.conda.io/projects/conda/en/latest/user-guide/cheatsheet.html
.. _PyFACT: https://ui.adsabs.harvard.edu/abs/2012AIPC.1505..789R
.. _healpy: https://healpy.readthedocs.io/en/latest/
.. _HEALPix: https://en.wikipedia.org/wiki/HEALPix
