docs: fix README override example and wording

[lesnik512]

Summary

README fixes from a docs review.

The Overrides example applied di_container.override(...) inside the test body, but user_service is a fixture that already resolved during setup (modern_di_pytest/factory.py, _fixture calls container.resolve_dependency eagerly). The assertion ran against the real repo. Reproduced as a pytest run: the old example fails with assert ['real'] == []; the new one passes.

Changes

• Overrides: override di_container in the test module and apply the override there (with di_container.override(...)), so generated fixtures resolve against it
• Replace the "3.1: graph validation is an explicit call now" comment with current behavior (the pin is modern-di>=3,<4)
• "The user owns the root container fixture" -> "You provide the root container fixture"
• Remove em dashes in the intro and API section (pyproject description untouched)
• Shared footer: "with an IoC container"; org link sentence without a dash

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging