Correct nested Map middleware example

[yermekyerden]

Summary

This PR corrects the nested Map middleware example.

The example currently calls app.Run(...) inside the nested branches. Since app is the root application builder, the terminal middleware is registered on the root pipeline instead of the nested branch.

The updated code uses level2AApp.Run(...) and level2BApp.Run(...), which matches the intended nested Map behavior.

Validation

Documentation-only change.

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Internal previews

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aspnetcore/fundamentals/middleware/index.md	ASP.NET Core Middleware

[wadepickett]

@yermekyerden, Thank very much for identifying this issue and fixing it! Greatly appreciated!!

The original code used app.Run(...) (root pipeline builder) instead of the branch-specific builders level2AApp.Run(...) and level2BApp.Run(...), which would cause incorrect runtime behavior inconsistent with the response table shown in the article.

[wadepickett]

Approved.