Fixing link to pytest assertions

Author: fhightower

README.rst

@@ -238,7 +238,7 @@ Note that this feature is automatically disabled with the ``--tb=native`` option
 mechanism used to suppress traceback entries from ``mock`` module does not work with that option
 anyway plus it generates confusing messages on Python 3.5 due to exception chaining
 
-.. _advanced assertions: http://docs.pytest.org/en/latest/assert.html
+.. _advanced assertions: http://docs.pytest.org/en/stable/assert.html
 
 
 Use standalone "mock" package