Typo fix. (#3833)

CONTRIBUTING.md

@@ -43,7 +43,7 @@ The preferred workflow for contributing to PyMC3 is to fork the [GitHub reposito
 
    Always use a ``feature`` branch. It's good practice to never routinely work on the ``master`` branch of any repository.
 
-4. Project requirements are in ``requirements.txt``, and libraries used for development are in ``requirements-dev.txt``.  To set up a development environment, you may (probably in a [virtual environment](https://docs.python-guide.org/dev/virtualenvs/) run:
+4. Project requirements are in ``requirements.txt``, and libraries used for development are in ``requirements-dev.txt``.  To set up a development environment, you may (probably in a [virtual environment](https://docs.python-guide.org/dev/virtualenvs/)) run:
 
    ```bash
    $ pip install -r requirements.txt