Correct testing instruction

Author: stephane-caron

CONTRIBUTING.md

@@ -19,6 +19,6 @@ The goal of this project is to facilitate loading and sharing of robot descripti
     - For example, the file name for the Kinova (maker) Gen2 (robot name) is ``gen2_description.py``.
     - Use the ``mj_description`` suffix for an MJCF description.
 4. **Listing:** Add the description metadata to the ``DESCRIPTIONS`` dictionary in ``_descriptions.py``.
-5. **Testing:** Check that all unit tests are successful by ``tox -e py``.
+5. **Testing:** Check that all unit tests are successful by ``tox``.
 6. **README:** Document the description's submodule name in the Descriptions section of the [README](README.md).
 7. **Changelog:** Write down the new model at the top of the [changelog](CHANGELOG.md).