Fix CONTRIBUTING.rst. This commit will be squashed soon.

Co-authored-by: Chris Warrick <kwpolska@gmail.com>

CONTRIBUTING.rst

@@ -66,7 +66,7 @@ Here are some guidelines about how you can contribute to Nikola:
 * **Try writing some tests** if possible — again, following existing tests is
   often easiest, and a good way to tell whether the feature you are modifying is
   easily testable.
-* **Test your code.** If you can, run the test suite with ``f tests/``
+* **Test your code.** If you can, run the test suite with ``pytest tests/``
   (you will need to install pytest and some other requirements, see
   ``requirements-tests.txt``). Alternatively, you can push, make a PR and wait
   for our CI to pick up and test your changes (but note that workflows for new