Fix incorrect dotnet CLI command in NUnit tutorial

[Copilot]

The NUnit unit testing tutorial contained an incorrect CLI command that would not work for most users. The tutorial showed:

dotnet reference add ../PrimeService/PrimeService.csproj

However, this "noun first" form only works in .NET 10 SDK and later. The majority of users are still on .NET 9 or earlier, which requires the "verb first" form:

dotnet add reference ../PrimeService/PrimeService.csproj

Changes made:

• Fixed the CLI command to use dotnet add reference (works for .NET 9 and earlier)
• Updated the link text to match the corrected command
• Added a note explaining that .NET 10+ users can use the "noun first" form
• Maintained the link to the reference documentation which explains both forms

This change ensures the tutorial works for the vast majority of users while providing clarity about version differences.

Fixes #46919.

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📄 File	🔗 Preview link
docs/core/testing/unit-testing-csharp-with-nunit.md	docs/core/testing/unit-testing-csharp-with-nunit