fix indentation in XmlTextWriter example (#4168)

Author: krwq

xml/System.Xml/XmlTextWriter.xml

@@ -51,10 +51,10 @@
 ```csharp  
 w.WriteStartElement("root");  
 w.WriteAttributeString("xmlns", "x", null, "urn:1");  
- w.WriteStartElement("item","urn:1");  
- w.WriteEndElement();  
- w.WriteStartElement("item","urn:1");  
- w.WriteEndElement();  
+w.WriteStartElement("item","urn:1");  
+w.WriteEndElement();  
+w.WriteStartElement("item","urn:1");  
+w.WriteEndElement();  
 w.WriteEndElement();  
 ```