docs: Corrections to Middleware page

[developerjamiu]

Summary

Fact-checked the Middleware page. One correction:

• The Handler typedef was documented as Future<Response> Function(Request) but Relic actually defines it as FutureOr<Result> Function(Request). Result is a sealed type with Response, Hijack, and WebSocketUpgrade subclasses, which is why the example's if (result is Response) narrowing is required.

Added a one-line note explaining Result so readers can see why the type check exists.

Test plan

• Render the page locally and confirm the typedef block reads correctly.

[Swiftaxe]

LGTM!