[Cupertino] Fix Navigation Demo BorderRadius

Samples/UIWidgetsGallery/demo/cupertino/cupertino_navigation_demo.cs

                                                             CupertinoButton.filled(
                                                                 minSize: 30.0f,
                                                                 padding: EdgeInsets.zero,
-                                                                borderRadius: BorderRadius.circular(32.0f),
+                                                                borderRadius: BorderRadius.circular(16.0f),
                                                                 child: new Icon(CupertinoIcons.ellipsis),
                                                                 onPressed: () => { }
                                                             )