Update TreeViewPage.xaml document icon example to use Sagoe icon symbol instead of using hardcoded .png

Author: GID0317

source/iNKORE.UI.WPF.Modern.Gallery/Pages/Controls/Windows/TreeViewPage.xaml

@@ -21,7 +21,7 @@
 
         <DataTemplate x:Key="FileTemplate">
             <StackPanel Orientation="Horizontal">
-                <Image Width="20" Source="/Assets/file.png" />
+                <ui:FontIcon Icon="{x:Static ui:SegoeFluentIcons.Document}" FontSize="20"/>
                 <TextBlock Margin="0,0,10,0" />
                 <TextBlock Text="{Binding Name}" />
             </StackPanel>