Fix typo 'tox' -> 'nox' in contributing docs
@@ -54,7 +54,7 @@ versions.::
Note that code coverage less than 100% is regarded as a failing run. Some
platform-specific tests are skipped unless run in that platform. To make sure
-the code works in all of urllib3's supported platforms, you can run our ``tox``
+the code works in all of urllib3's supported platforms, you can run our ``nox``
suite::
$ nox --reuse-existing-virtualenvs --sessions test
