Update mainwindow.md (#10506)

Missing comma between line 17 and 18

Author: BcnCarlos

articles/quickstart/native/wpf-winforms/files/mainwindow.md

@@ -20,7 +20,7 @@ public partial class MainWindow : Window
     {
         Auth0ClientOptions clientOptions = new Auth0ClientOptions
         {
-            Domain = "${account.namespace}"
+            Domain = "${account.namespace}",
             ClientId = "${account.clientId}"
         };
         client = new Auth0Client(clientOptions);