使用XmlDocument创建保存XML文件，使用txt或记事本打开后XML格式问题

XmlDocument doc = new XmlDocument();
Doc = …;
string s = doc.OuterXml;
byte[] MyData = new byte[0];
SaveFileDialog savetxtdialog = new SaveFileDialog();
savetxtdialog.Filter = "XML文件(*.xml)|*.xml";
savetxtdialog.RestoreDirectory = true;
savetxtdialog.FileName = @"C:\" + zdh + ".xml";
DialogResult dialogResult = savetxtdialog.ShowDialog(this);
if (dialogResult == DialogResult.OK)
{
                MyData = System.Text.Encoding.Default.GetBytes(s);

                string filePath = savetxtdialog.FileName;
                FileStream dgnfs = new FileStream(filePath, FileMode.Create, FileAccess.Write);
                dgnfs.Write(MyData, 0, MyData.Length);

                //清空缓冲区、关闭流
                dgnfs.Flush();
              dgnfs.Close();
}
XmlDocument doc = new Xml