Fix incorrect delegate type name in using-delegates.md

[satanmoo]

The document declares a delegate named Callback:

public delegate void Callback(string message);

However, later in the document it incorrectly refers to it as Del:

"we now have three methods that you can wrap in a Del instance"

This should be Callback to match the actual delegate declaration used throughout the example.

Summary

Fixed an inconsistency in the delegate documentation where Del was incorrectly used instead of Callback.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/delegates/using-delegates.md	Using Delegates (C# Programming Guide)

[BillWagner]

Thanks for catching this @satanmoo

This looks great, and I'll now. You should see the changes on the live site in the next day or two, on our regular publishing cycle.

[satanmoo]

Thanks for the heads-up! I'm looking forward to seeing the changes live on the site.