[FIX] web_editor: fix placeholders for multi editor

Fix placeholder not working properly when multiple editor exists

task-2637488

Part-of: odoo/odoo#83792

addons/web_editor/static/lib/odoo-editor/src/OdooEditor.js

@@ -2320,7 +2320,7 @@ export class OdooEditor extends EventTarget {
 
     clean() {
         this.observerUnactive();
-        for (const hint of this.document.querySelectorAll('.oe-hint')) {
+        for (const hint of this.editable.querySelectorAll('.oe-hint')) {
             hint.classList.remove('oe-hint', 'oe-command-temporary-hint');
             hint.removeAttribute('placeholder');
         }