Fixing typos

I fixed some typos I noticed and added underscores to have __main__.py instead of main.py in the depiction of what the project structure should look like to make this match the explanation (see commit). I hope those were not left out for any reason I'm currently missing.

added Bug::Typo label

requested review from @erxleb87

approved this merge request

Thanks, It is really interesting to see how many typos there still are in this bit of content

merged

mentioned in commit f67526bc