Fix tox env used for flake8 in contributing docs (#1192)

The tox environment to run flake8 changed in 6af081c.

docs/contributing.rst

@@ -263,7 +263,7 @@ Try reading our code and grasp the overall philosophy regarding method and varia
 the sake of readability, keep in mind that *simple is better than complex*. If you feel the code is not straightforward,
 add a comment. If you think a function is not trivial, add a docstrings.
 
-To see if your code formatting will pass muster use: `tox -e py37-flake8`
+To see if your code formatting will pass muster use: `tox -e flake8`
 
 
 The contents of this page are heavily based on the docs from `django-admin2 <https://github.com/twoscoops/django-admin2>`_