Update repository-specific guidelines in instructions

[guardrex]

A quick patch that I don't think rises to the level of requiring an issue.

This came about because Copilot deliberately tried to remove a contraction from a release notes update ...

#36224 (comment)

I edited Copilot's suggested change to put the contraction back.

Cross-ref: Use contractions

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Internal previews

📄 File	🔗 Preview link
.github/copilot-instructions.md	Copilot Instructions for dotnet/AspNetCore.Docs

[Copilot]

Pull Request Overview

This PR adds an explicit guideline for Copilot to use contractions when writing documentation, aligning with Microsoft Writing Style Guide best practices. This addresses an issue where Copilot was incorrectly removing contractions from documentation updates.

Key change:

• Added a nested guideline under the Microsoft Writing Style Guide section to explicitly instruct Copilot to use contractions per Microsoft's style guidance

[wadepickett]

Looks good.

[guardrex]

Thanks @wadepickett!