doc: made example pep8 complaint

doc/sources/guide/widgets.rst

@@ -368,41 +368,37 @@ Pure Python way::
     from kivy.graphics import Color, Rectangle
     from kivy.uix.floatlayout import FloatLayout
     from kivy.uix.button import Button
-
-
+    
+    
     class RootWidget(FloatLayout):
-
+    
         def __init__(self, **kwargs):
             # make sure we aren't overriding any important functionality
             super(RootWidget, self).__init__(**kwargs)
-
+    
             # let's add a Widget to this layout
             self.add_widget(
-                            Button(
-                                    text="Hello World",
-                                    size_hint= (.5, .5),
-                                    pos_hint={'center_x':.5,
-                                            'center_y':.5}))
-
-
+                Button(
+                    text="Hello World",
+                    size_hint=(.5, .5),
+                    pos_hint={'center_x': .5, 'center_y': .5}))
+    
+    
     class MainApp(App):
-
+    
         def build(self):
             self.root = root = RootWidget()
-            root.bind(
-                        size=self._update_rect,
-                        pos=self._update_rect)
-            with root.canvas.before:
-                Color(0, 1, 0, 1) # green; colors range from 0-1 not 0-255
-                self.rect = Rectangle(
-                                        size=root.size,
-                                        pos=root.pos)
-            return root
+            root.bind(size=self._update_rect, pos=self._update_rect)
 
+            with root.canvas.before:
+                Color(0, 1, 0, 1)  # green; colors range from 0-1 not 0-255
+                self.rect = Rectangle(size=root.size, pos=root.pos)
+            return root
+    
         def _update_rect(self, instance, value):
             self.rect.pos = instance.pos
             self.rect.size = instance.size
-
+    
     if __name__ == '__main__':
         MainApp().run()