Code block fix (dotnet#2513)

* Code block fix

One of the code blocks was a bit weirdly formatted.

* Update ControlTemplate.xml

Author: TimoSalomaki

xml/System.Windows.Controls/ControlTemplate.xml

@@ -39,10 +39,11 @@
 <a name="xamlObjectElementUsage_VisualTree"></a>   
 ## XAML Property Element Usage  
   
-```  
+```xaml
 <ControlTemplate>  
-  <VisualTreeRootNode > VisualTreeNodeContents  
-  </VisualTreeRootNode >  
+  <VisualTreeRootNode>
+    VisualTreeNodeContents  
+  </VisualTreeRootNode>  
 </ControlTemplate>  
 ```  
   
@@ -319,4 +320,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>