Updated Pytest example in upgrade documentation.

Pytest changed their recommended entrypoint to 'pytest' in version 3.0.0 in 2016 : https://docs.pytest.org/en/latest/changelog.html#id1009. Also link the name since this is the only reference in the Django documentation at current.

docs/howto/upgrade-version.txt

@@ -68,11 +68,11 @@ environment variable. For example, to show warnings while running tests:
 
 If you're not using the Django test runner, you may need to also ensure that
 any console output is not captured which would hide deprecation warnings. For
-example, if you use `py.test`:
+example, if you use `pytest <https://pytest.org/>`__:
 
 .. code-block:: console
 
-    $ PYTHONWARNINGS=always py.test tests --capture=no
+    $ PYTHONWARNINGS=always pytest tests --capture=no
 
 Resolve any deprecation warnings with your current version of Django before
 continuing the upgrade process.