Fix the ruff include glob so example/ is actually linted

[Danipulok]

What

Point the ruff include glob at example/, and add example/test_example.py to exclude.

Why

The glob names examples/, but the directory is example/, so it has never matched anything and the example package the suite runs against is not linted at all.

That file is the one thing under example/ that cannot be linted or formatted, because it is its own test data: find_examples parses its docstrings. docstring-code-format rewrites the #> print markers inside them to # >, and D212 would collapse the two docstring styles that test_python_self_change_docstyle exists to cover. It joins tests/cases_update/*.py in exclude, which already holds test data for the same reason.

How to test

make lint passes and leaves example/test_example.py untouched. Add any other .py file under example/ and ruff check now reports it.