Added the missing XML tag (#3342)

Author: TimoSalomaki

xml/System.Windows.Forms/TreeView.xml

@@ -59,7 +59,7 @@
   
  The state images displayed in the <xref:System.Windows.Forms.TreeView> are 16 x 16 pixels by default. Setting the <xref:System.Windows.Forms.ImageList.ImageSize%2A> property of the <xref:System.Windows.Forms.TreeView.StateImageList%2A> will have no effect on how the images are displayed. However, the state images are resized according to the system DPI setting when the app.config file contains the following entry:  
   
-```  
+```xml  
 <appSettings>  
   <add key="EnableWindowsFormsHighDpiAutoResizing" value="true" />  
 </appSettings>