Update example to replace broken link and reference to `SKFunction` attribute

[HillPhelmuth]

C# Example in Agents - Plugins section was still using old SKFunction attribute and link was to the old DocumentSkill folder. Updated both.

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

Learn Build status updates of commit 0a13405:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
semantic-kernel/agents/plugins/index.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel