Fix ProjectReference path in preprocessor directives

[AdmiralSnyder]

those two samples should refer to the same folder structure. which they did not, previously.

Reasoning

i.e. the in the .csproj needs to reference the target project's .csproj
whereas the #:project entry in the .cs references the folder the project is in.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/preprocessor-directives.md	C# preprocessor directives

[Copilot]

Pull Request Overview

This PR corrects a documentation inconsistency in the preprocessor directives guide by fixing the ProjectReference path example to properly reflect the folder structure used in the #:project directive example.

Key Changes

• Updated the XML ProjectReference path to include the project folder in its path structure, ensuring consistency with the #:project directive example that references the folder containing the project.

[BillWagner]

Thanks @AdmiralSnyder

This LGTM. I'll now.