Use ruff.toml format in README (#10393)
## Summary
See feedback in
https://github.com/astral-sh/ruff/issues/4725#issuecomment-1994615409.
In the docs, we use a tabbed interface for express `ruff.toml` vs.
`pyproject.toml`. Here, it might be clearer to default to `ruff.toml`,
since it's more obviously _not_ `pyproject.toml`. But either way, this
PR attempts to clarify that there's a difference.
