docs: use src layout paths in contributor commands

[raghav-sharma-ai]

The contributor guide still points its Black and slow-test examples at a top-level trio directory. The package now lives at src/trio, so copying those commands from the repository root fails before checking any files. Update the three commands and the accompanying package-path wording.

Validation:

• The original Black command fails with Path 'trio' does not exist.
• black --check --diff --workers 1 src/trio succeeds (143 files unchanged).
• sphinx-lint docs/source/contributing.rst and git diff --check pass.

Documentation-only change; the full slow-test suite was not run. Prepared with AI assistance (OpenAI Codex).

[A5rocks]

CI failing isn't due to this change. However, I think in general we should rewrite the contributor docs, to e.g. point at pre-commit instead of black, and I don't think we need the package to pytest? Probably and other stuff.

But I'd like the contributor documentation not to be LLM generated (that would just be rude!), so maybe someone else can do this!