syntax highlighting of c# example code (#3261)

fixed c# syntax highlighting

Author: stesee

xml/System.Xml.Linq/XElement.xml

@@ -420,8 +420,7 @@ Console.WriteLine(el)
   
  The following example creates an element in a namespace with no content. For more information, see [Working with XML Namespaces](https://msdn.microsoft.com/library/e3003209-3234-45be-a832-47feb7927430).  
   
-```  
-
+```csharp
 XNamespace aw = "http://www.adventure-works.com";  
 XElement root = new XElement(aw + "Root");  
 Console.WriteLine(root);