fix for refreshing layout

docs/examples/widgets/placeholders.py

@@ -1,4 +1,3 @@
-from textual import events
 from textual.app import App
 from textual.widgets import Placeholder
 
@@ -6,7 +5,7 @@ from textual.widgets import Placeholder
 class SimpleApp(App):
     """Demonstrates smooth animation"""
 
-    async def on_mount(self, event: events.Mount) -> None:
+    async def on_mount(self) -> None:
         """Build layout here."""
 
         await self.view.dock(Placeholder(), edge="left", size=40)