Update ProgramDesign

ProgramDesign.md

@@ -56,7 +56,9 @@ ExitButton.Click += ExitClick;
 ExitButton.MouseOver = ExitMouseOver;
 ExitButton.MouseLeave = ExitMouseLeave;
 ExitButton.SetName(“ExitButton”);
+```
 Now, let’s make our different functions.
+```c#
 public void ExitClick(object sender, SpriteEventArgs e)
 {
     If(sender is Sprite)