[Python] QHelp references old-ish python 2 documentation

I was looking at the [py/modification-of-default-value](https://codeql.github.com/codeql-query-help/python/py-modification-of-default-value/) query and noticed that it still refers to python 2 documentation.
A quick search reveals that there are more such references:
https://github.com/search?q=repo%3Agithub%2Fcodeql+%22python.org%2F2%22+language%3AXML&type=code&l=XML

While python 2.7 is still officially supported by CodeQL I think it would make sense to update those references to python 3.
But I of course also understand that this is probably a low priority issue :)

Start with the py/modification-of-default-value query-help page and the provided repository search for python.org/2 references in XML. Review each matching reference and update applicable Python 2 documentation links to Python 3. Done means the outdated references identified by the search have been updated without changing references that still support Python 2.

Written by the indexing model from the issue text.