Revert example code to composing the old way

We've not documented it yet so, until we do...

docs/examples/widgets/checkbox.py

@@ -7,15 +7,16 @@ class CheckboxApp(App[None]):
     CSS_PATH = "checkbox.css"
 
     def compose(self) -> ComposeResult:
-        with Vertical():
-            yield Checkbox("Arrakis :sweat:")
-            yield Checkbox("Caladan")
-            yield Checkbox("Chusuk")
-            yield Checkbox("[b]Giedi Prime[/b]")
-            yield Checkbox("[magenta]Ginaz[/]")
-            yield Checkbox("Grumman", True)
-            yield Checkbox("Kaitain", id="initial_focus")
-            yield Checkbox("Novebruns", True)
+        yield Vertical(
+            Checkbox("Arrakis :sweat:"),
+            Checkbox("Caladan"),
+            Checkbox("Chusuk"),
+            Checkbox("[b]Giedi Prime[/b]"),
+            Checkbox("[magenta]Ginaz[/]"),
+            Checkbox("Grumman", True),
+            Checkbox("Kaitain", id="initial_focus"),
+            Checkbox("Novebruns", True),
+        )
 
     def on_mount(self):
         self.query_one("#initial_focus", Checkbox).focus()