Fix typos in documentation & code

[katrinleinweber]

Essentially the same as adamtornhill/code-maat#64 plus d11f3c6 might even be fixing an error. That script does not seem to be used in the book, though, so I'm not sure how check the typo's & fix' real-world effect. Looks like an issue ;-)

[adamtornhill]

Thanks again -- this is great!

Yes, that looks like a real bug. However, that file git_complexity_diff.py was more of an early experiment to dive deeper into code complexity trends. The right thing is most likely to delete it.

[katrinleinweber]

You're welcome & thanks for merging :-)