Update xml-config-file.md (#943)

Add missing / to service account block.

docs/xml-config-file.md

@@ -412,7 +412,7 @@ Optional. Prompts for a user name and a password.
 ```xml
 <serviceaccount>
   <prompt>dialog|console</prompt>
-<serviceaccount>
+</serviceaccount>
 ```
 
 - `dialog`