Fix padding inconsistency

The padding is always added to the side the headers are at, except when the headers are right, when the padding is at the bottom. I believe this was a typo.

src/Avalonia.Themes.Default/TabControl.xaml

@@ -57,6 +57,6 @@
         <Setter Property="Orientation" Value="Vertical"/>
     </Style>
     <Style Selector="TabControl[TabStripPlacement=Right]">
-        <Setter Property="Padding" Value="0 0 0 4"/>
+        <Setter Property="Padding" Value="0 0 4 0"/>
     </Style>
 </Styles>