fix: replace scrollableBuilder with builder in CupertinoSheetRoute for simplified sheet implementation

Author: Pranav2918

cupertino_gallery/lib/widgets/sheet_page.dart

@@ -13,25 +13,20 @@ class SheetPage extends StatelessWidget {
           onPressed: () {
             Navigator.of(context).push(
               CupertinoSheetRoute<void>(
-                scrollableBuilder:
-                    (BuildContext context, ScrollController controller) {
-                      Widget widgetBuilder(BuildContext context) {
-                        return CupertinoPageScaffold(
-                          navigationBar: CupertinoNavigationBar(
-                            middle: const Text('Sheet'),
-                            trailing: GestureDetector(
-                              child: const Icon(CupertinoIcons.xmark),
-                              onTap: () {
-                                Navigator.of(context).pop();
-                              },
-                            ),
-                          ),
-                          child: const Center(child: Text('This is a sheet')),
-                        );
-                      }
-
-                      return widgetBuilder(context);
-                    },
+                builder: (BuildContext context) {
+                  return CupertinoPageScaffold(
+                    navigationBar: CupertinoNavigationBar(
+                      middle: const Text('Sheet'),
+                      trailing: GestureDetector(
+                        child: const Icon(CupertinoIcons.xmark),
+                        onTap: () {
+                          Navigator.of(context).pop();
+                        },
+                      ),
+                    ),
+                    child: const Center(child: Text('This is a sheet')),
+                  );
+                },
               ),
             );
           },