Example of modifying XML

issue:https://gitee.com/openharmony/docs/issues/I60CSXSigned-off-by: N小马奔腾 <maxiaodong16@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-xml.md

@@ -58,7 +58,9 @@ setAttributes(name: string, value: string): void
 let arrayBuffer = new ArrayBuffer(1024);
 let bufView = new DataView(arrayBuffer);
 let thatSer = new xml.XmlSerializer(bufView);
+thatSer.startElement("note");
 thatSer.setAttributes("importance", "high"); 
+thatSer.endElement(); 
 ```