docs: fix README footer wording

[lesnik512]

Summary

Shared README footer wording fix across the modern-di integrations.

Changes

• 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