Changed mentioned callback name from 'Del' to 'MultiplyCallback' (#43036)

Author: batkaevruslan

docs/csharp/programming-guide/delegates/delegates-with-named-vs-anonymous-methods.md

@@ -29,7 +29,7 @@ var write = Console.Write; // ERROR: Multiple overloads, can't choose
 
 ## Examples
 
-The following is a simple example of declaring and using a delegate. Notice that both the delegate, `Del`, and the associated method, `MultiplyNumbers`, have the same signature
+The following is a simple example of declaring and using a delegate. Notice that both the delegate, `MultiplyCallback`, and the associated method, `MultiplyNumbers`, have the same signature
 
 [!code-csharp[csProgGuideDelegates#2](~/samples/snippets/csharp/VS_Snippets_VBCSharp/csProgGuideDelegates/CS/Delegates.cs#2)]