Actually use the interface in the example (#6077)

Seems like an interface was created but the actual usage example did not use it. Instead, it used the concrete type.

Author: ernestochaves

docs/csharp/language-reference/keywords/codesnippet/CSharp/interface_2.cs

@@ -63,9 +63,9 @@ static void PrintPoint(IPoint p)
 
        static void Main()
        {
-          Point p = new Point(2, 3);
+          IPoint p = new Point(2, 3);
           Console.Write("My Point: ");
           PrintPoint(p);
        }
     }
-    // Output: My Point: x=2, y=3
+    // Output: My Point: x=2, y=3