Fix BindingDemo

samples/BindingDemo/MainWindow.xaml

@@ -6,7 +6,7 @@
         Title="AvaloniaUI Bindings Test"
         Width="800"
         Height="600"
-        x:DataContextType="{x:Type vm:MainWindowViewModel}">
+        x:DataContextType="vm:MainWindowViewModel">
   <Window.Styles>
     <Style Selector="TextBlock.h1">
       <Setter Property="FontSize" Value="18"/>