Add information on where to run code in README

The README mentions that one should switch to the code/ dir in one section, but this info should also be there in the "How to run" section, especially as "the same directory" is referenced.

https://gitlab.com/hifis/hifis-workshops/make-your-code-ready-for-publication/astronaut-analysis/-/blob/master/README.md#L65

Suggestions: change to

cd code
python astronauts-analysis.py