Remove reference to building conda env from yml file

Since the guide isn't using the environment.yml file anymore
it doesn't make a lot of sense to link to a doc about building
a conda environment from a yml file.

Author: mriedem

CONTRIBUTING.rst

@@ -171,17 +171,14 @@ Building the Documentation
 To build the documentation you'll need `Sphinx <http://www.sphinx-doc.org/>`_,
 `pandoc <http://pandoc.org/>`_ and a few other packages.
 
-To install (and activate) a `conda environment`_ named ``notebook_docs``
+To install (and activate) a conda environment named ``notebook_docs``
 containing all the necessary packages (except pandoc), use::
 
     conda create -n notebook_docs pip
     conda activate notebook_docs  # Linux and OS X
     activate notebook_docs        # Windows
     pip install .[docs]
 
-.. _conda environment:
-    https://conda.io/docs/user-guide/tasks/manage-environments.html#creating-an-environment-from-an-environment-yml-file
-
 If you want to install the necessary packages with ``pip``, use the following instead::
 
     pip install .[docs]