docs: fix WebSocketEndpoint example scope error

[lesnik512]

Summary

Fix the WebSocketEndpoint example, which raised ScopeNotInitializedError.

Changes

• The Echo.on_receive example injected Dependencies.user_service, a REQUEST-scoped provider. A WebSocket connection gets a SESSION-scoped child (starlette_websocket_provider, modern_di_starlette/main.py:18), so resolution failed with "Provider of scope REQUEST cannot be resolved in container of scope SESSION". It now injects the APP-scoped Dependencies.settings, and the scopes paragraph says WebSocket handlers can inject APP and SESSION dependencies but not REQUEST ones.
• Removed an em dash in the setup_di note.
• Footer: "with IoC container" -> "with an IoC container"; replaced the em dash in the org link line.

Verified with a scratch app (TestClient): the old example fails with the error above; the new one echoes over the WebSocket and the HTTPEndpoint route still returns 200.

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