fix merlin demo

Author: willmcgugan

examples/merlin.py

@@ -163,7 +163,7 @@ def on_switch_changed(self, event: Switch.Changed) -> None:
                 self.query_one(f"#switch-{toggle_no}", Switch).toggle()
         # Check the win
         if self.check_win():
-            self.query_one("Screen").add_class("-win")
+            self.screen.add_class("-win")
             self.query_one(Timer).running = False
             self.notify("You win!", title="congratulations", severity="information")