Remove mutable default arg in function example (#944)

* Remove mutable default arg in function example

Removed mutable default arg, replaced with `None`.
https://docs.quantifiedcode.com/python-anti-patterns/correctness/mutable_default_value_as_argument.html

Replaced `dict()` with `{}`.
https://stackoverflow.com/questions/8424942/creating-a-new-dictionary-in-python

* Wrong import

* Undo last commit

My bad.

Co-authored-by: Hynek Schlawack <hs@ox.cx>
1 file changed