Update XmlNode.xml (dotnet#9113)

The example output contains an erroneous space: <markup /> should be <markup/>

Author: zacharylayne

xml/System.Xml/XmlNode.xml

@@ -1042,7 +1042,7 @@ Display the InnerXml of the element...
 some text<child />more text  
 <elem>Text containing &lt;markup/&gt; will have char(&lt;) and char(&gt;) escape  
 d.</elem>  
-<elem>Text containing <markup />.</elem>  
+<elem>Text containing <markup/>.</elem>  
 ```  
   
  ]]></format>