Fix warnings in example

example/lib/widgets/demo_scaffold.dart

@@ -19,12 +19,12 @@ class DemoScaffold extends StatefulWidget {
   final bool showToolbar;
 
   const DemoScaffold({
-    Key? key,
     required this.documentFilename,
     required this.builder,
     this.actions,
     this.showToolbar = true,
     this.floatingActionButton,
+    Key? key,
   }) : super(key: key);
 
   @override
@@ -59,14 +59,14 @@ class _DemoScaffoldState extends State<DemoScaffold> {
       final doc = Document.fromJson(jsonDecode(result));
       setState(() {
         _controller = QuillController(
-            document: doc, selection: TextSelection.collapsed(offset: 0));
+            document: doc, selection: const TextSelection.collapsed(offset: 0));
         _loading = false;
       });
     } catch (error) {
       final doc = Document()..insert(0, 'Empty asset');
       setState(() {
         _controller = QuillController(
-            document: doc, selection: TextSelection.collapsed(offset: 0));
+            document: doc, selection: const TextSelection.collapsed(offset: 0));
         _loading = false;
       });
     }
@@ -97,7 +97,7 @@ class _DemoScaffoldState extends State<DemoScaffold> {
       ),
       floatingActionButton: widget.floatingActionButton,
       body: _loading
-          ? Center(child: Text('Loading...'))
+          ? const Center(child: Text('Loading...'))
           : widget.builder(context, _controller),
     );
   }