Fix for C# example as original did not run

[graysddoman]

The C# example did not work as expected, so have updated the document to include details for the libraries used and some changes to the code in order for the example to work as expected.

[prmerger-automator]

@graysddoman : Thanks for your contribution! The author(s) have been notified to review your proposed change. @aapowell

[learn-build-service-prod]

Learn Build status updates of commit 0618832:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/ai-services/openai/includes/assistants-csharp.md	✅Succeeded

For more details, please refer to the build report.

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[ttorble]

@aaronpowell

Can you review the proposed changes? The "note" at lines 74-76 will need to be formatted by using the custom Markdown for alerts before the PR can be merged.

Note that the author metadata should reflect your GitHub alias. Can you create a PR in the private repo to update this? Thanks.

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team