README: update the tox link

sa-flxc · sa-flxc · commit 598d278e87a6 · 2017-10-02T15:22:37.000+02:00
diff --git a/README.rst b/README.rst
@@ -732,7 +732,7 @@ See ``tox.ini`` file to learn how to use ``nosetests`` directly to
 test individual Python versions.
 
 .. _nose: https://nose.readthedocs.org/
-.. _tox: http://tox.testrun.org/
+.. _tox: https://tox.readthedocs.io/
 
 
 Contributors
