[FIX] web_editor: xml editor first level not shown

There is a slight visual inconsistency since: 9af8d5e6 the xml/less
editor for first level of descendants were just prefixed by an invisible
space.

opw-807624
closes #22704

addons/web_editor/static/src/js/widgets/ace.js

@@ -742,7 +742,7 @@ var ViewEditor = Widget.extend({
 
             var text = data.text || '';
             if (!isSelected) {
-                text = Array($elem.data('level')).join('-') + ' ' + text;
+                text = Array($elem.data('level') + 1).join('-') + ' ' + text;
             }
             var $div = $('<div/>',  {
                 text: text,