Fixed broken link C# Core Skills

[JimPaine]

link to C# core plugins was broken. I have updated it to point to the latest location

[learn-build-service-prod[bot]]

Learn Build status updates of commit b5e0ae2:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
semantic-kernel/ai-orchestration/out-of-the-box-plugins.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

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[matthewbolanos]

Thanks for the fix!