Typo in Packaging tutorial

Thanks for this nice collection of wisdom!

In the Packaging tutorial [here](https://learn.scientific-python.org/development/tutorials/packaging/#create-a-minimal-installable-python-package), the `__init__.py` and `refraction.py` files are placed in the wrong directory, under `src` instead of `src/example`.  The `tree` output of the file structure is correct, however.