Fix: DataGrid Grouping symbol wrong in Fluent Theme

- Expanded icon was vertically flipped.
- ToggleButton :checked means open, :unchecked  means closed. The icon reference was corrected here to match this.

src/Avalonia.Controls.DataGrid/Themes/Fluent.xaml

@@ -7,7 +7,7 @@
     <StreamGeometry x:Key="DataGridSortIconDescendingPath">M1875 1011l-787 787v-1798h-128v1798l-787 -787l-90 90l941 941l941 -941z</StreamGeometry>
     <StreamGeometry x:Key="DataGridSortIconAscendingPath">M1965 947l-941 -941l-941 941l90 90l787 -787v1798h128v-1798l787 787z</StreamGeometry>
     <StreamGeometry x:Key="DataGridRowGroupHeaderIconClosedPath">M515 93l930 931l-930 931l90 90l1022 -1021l-1022 -1021z</StreamGeometry>
-    <StreamGeometry x:Key="DataGridRowGroupHeaderIconOpenedPath">M1939 1581l90 -90l-1005 -1005l-1005 1005l90 90l915 -915z</StreamGeometry>
+    <StreamGeometry x:Key="DataGridRowGroupHeaderIconOpenedPath">M109 486 19 576 1024 1581 2029 576 1939 486 1024 1401z</StreamGeometry>
 
     <SolidColorBrush x:Key="DataGridColumnHeaderForegroundBrush" Color="{DynamicResource SystemBaseMediumColor}" />
     <SolidColorBrush x:Key="DataGridColumnHeaderBackgroundBrush" Color="{DynamicResource SystemAltHighColor}" />
@@ -499,12 +499,12 @@
   </Style>
 
   <Style Selector="DataGridRowGroupHeader /template/ ToggleButton#ExpanderButton /template/ Path">
-    <Setter Property="Data" Value="{StaticResource DataGridRowGroupHeaderIconOpenedPath}" />
+    <Setter Property="Data" Value="{StaticResource DataGridRowGroupHeaderIconClosedPath}" />
     <Setter Property="Stretch" Value="Uniform" />
   </Style>
 
   <Style Selector="DataGridRowGroupHeader /template/ ToggleButton#ExpanderButton:checked /template/ Path">
-    <Setter Property="Data" Value="{StaticResource DataGridRowGroupHeaderIconClosedPath}" />
+    <Setter Property="Data" Value="{StaticResource DataGridRowGroupHeaderIconOpenedPath}" />
     <Setter Property="Stretch" Value="UniformToFill" />
   </Style>