added syntax highlighting of xml examples (#3262)

language was missing, added "xml" after ```

Author: stesee

xml/System.Xml.Linq/XElement.xml

@@ -768,7 +768,7 @@ Console.WriteLine(root)
   
  This example produces the following output:  
   
-```  
+```xml
 <Root>Some text</Root>  
 <Root>  
   <NewChild>n</NewChild>  
@@ -1090,7 +1090,7 @@ Console.WriteLine(root)
   
  This example produces the following output:  
   
-```  
+```xml
 <Root>Some text</Root>  
 <Root>  
   <NewChild>n</NewChild>  
@@ -6579,7 +6579,7 @@ Console.WriteLine(root)
   
  This example produces the following output:  
   
-```  
+```xml  
 <Root>  
   <NewChild>n</NewChild>  
 </Root>