Update pr_tutorial.md

Make the python version consistent on the PR tutorial.

Author: juanitorduz

docs/source/contributing/pr_tutorial.md

@@ -34,7 +34,7 @@ The preferred workflow for contributing to PyMC is to fork the [GitHub repositor
    :::{tab-item} Linux/MacOS
 
    ```bash
-   conda env create -f conda-envs/environment-dev-py39.yml  # or py3x
+   conda env create -f conda-envs/environment-dev-py38.yml  # or py3x
    ```
    :::
    :::{tab-item} Windows