docs: Fixed example due to incorrect paths

[Carsopre]

Issue addressed

Solves #440

Code of conduct

• [x] I HAVE NOT added sensitive or compromised (test) data to the repository.
• [x] I HAVE NOT added vulnerabilities to the repository.
• [ ] I HAVE discussed my solution with (other) members of the RA2CE team.

What has been done?

Correct relative paths throughout the notebook. The previous implementation would only be valid when running under a windows environment.

Checklist

• [x] Code is formatted using our custom black and isort definitions.
• [x] Tests are either added or updated.
• [x] Branch is up to date with master.
• [x] Updated documentation if needed.

Additional Notes (optional)

Added explicit joinpath instead of / for Pathlib.Path objects.

[Carsopre]

I would expect this will lead to problems with all/most other notebooks as well, as they also use "/" to construct paths.

That's not true, the / is accepted to join paths, .joinpath is simply more explicit and therefore why I modified this one. The jupyter notebooks are standalone examples and it's natural they will differentiate one from the other as code will change with time.