use existing ref

Author: willingc

source/guides/installing-using-virtualenv.rst

@@ -6,11 +6,9 @@ This guide discusses how to install packages using :ref:`pip` and
 
 .. important::
     This "how to" guide on installing packages and using :ref:`virtualenv` is
-    under development. Please refer to the `virtualenv's documentation`_ for
+    under development. Please refer to the :ref:`virtualenv` documentation for
     details on installation and usage.
 
-    .. virtualenv's documentation: https://virtualenv.pypa.io/
-
 
 .. note:: This doc uses the term **package** to refer to a
     :term:`Distribution Package`  which is different from an :term:`Import