Sync README tutorial with example package

[Herrtian]

Fixes #32.

The README tutorial had drifted from the checked-in example/mypkg package: it introduced path, Class1, and clsattr, but later showed mypkg.sub, which does not exist in that snippet.

This updates the README tutorial to use the same SomeClass / sub.OtherClass layout as the example package and updates the import snippet to match.

Checked with:

• git diff --check
• PYTHONPATH=example python -c "import mypkg; print(mypkg.SomeClass); print(mypkg.sub); print(mypkg.sub.OtherClass)"
• python -m pytest -q (44 passed, after installing pytest and setuptools in a local venv)

[Herrtian]

Heads up: pre-commit.ci is failing in the existing pyupgrade hook before it gets to anything related to this README-only change:

TypeError: cannot use a bytes pattern on a string-like object

It looks like the old pyupgrade hook is running under Python 3.14 on pre-commit.ci. I left this PR scoped to the README fix for #32 rather than mixing in a pre-commit config update.