docs: fix hyperlink target syntax in CONTRIBUTING.rst

Author: chalmerlowe

CONTRIBUTING.rst

@@ -73,7 +73,7 @@ We use `nox <https://nox.readthedocs.io/en/latest/>`__ to instrument our tests.
 
     The unit tests and system tests are described in the ``noxfile.py`` files in each package directory.
 
-.. nox: https://pypi.org/project/nox/
+.. _nox: https://pypi.org/project/nox/
 
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 I'm getting weird errors... Can you help?