Update link to Community Toolkit Contributing.md

[enitimeago]

What changes to the docs does this PR provide?

This PR updates the link to the Windows Community Toolkit's contributing docs. The link at time of writing leads to a 404,

PR Checklist

Please check if your PR fulfills the following requirements:

• Correctly picked the right branch to base the change off (dev for new features, main for typos/improvements)
• For new pages, used the provided template
• For new features, added an entry in the Table of Contents
• Ran against a spell and grammar checker
• Contains NO breaking changes

Other information

[learn-build-service-prod]

Learn Build status updates of commit 5f2f792:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
docs/docfx.json	⚠️Warning		Details
dotnet/docfx.json	⚠️Warning		Details
CONTRIBUTING.md	✅Succeeded

docs/docfx.json

• Line 36, Column 50: [Warning: ms-prod-technology-invalid - See documentation] Invalid value for 'ms.technology': 'windows-community-toolkit' is not valid with 'ms.prod' value 'dotnet-communitytoolkit'.

dotnet/docfx.json

• Line 54, Column 50: [Warning: ms-prod-technology-invalid - See documentation] Invalid value for 'ms.technology': 'windows-community-toolkit' is not valid with 'ms.prod' value 'dotnet-communitytoolkit'.

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[michael-hawker]

Thanks! Still keep finding these old links from the old repo name...