docs: fix FromDI facts in README

[lesnik512]

Summary

Fix README statements that do not match the code.

Changes

• Usage intro said setup_di "builds a per-connection child container automatically". setup_di (modern_di_fastapi/main.py:54-58) only stores the container, registers the context providers and composes the lifespan. The child is built on demand by Depends(build_di_container) inside FromDI (main.py:74-77), so only routes that use FromDI get one.
• API table: FromDI(provider, ...) -> FromDI(dependency, ...), matching the signature at main.py:80.
• API table: removed an em dash in the build_di_container row; added the scope to the fastapi_request_provider (REQUEST) and fastapi_websocket_provider (SESSION) rows (main.py:14-15), as the aiohttp and starlette READMEs do.
• Footer: "with IoC container" -> "with an IoC container"; replaced the em dash in the org link line.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered (docs only, no behavior change)
• 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