[FIX] web_editor: fix iframe tree structure

When the debug assets option is not activated, the assets are rendered
with a space being trimmed from a string notation using backtick.
This fix, uses the single quote notation to avoid this issue in
`wysiwyg_iframe`. Without that fix, the structure of the dom is wrong
and causes css rules to not be applied.

task-3100202

X-original-commit: c54fcf21aecccac20b2c1d781132ed67df6c20a7
Part-of: odoo/odoo#112908

addons/web_editor/static/src/js/wysiwyg/wysiwyg_iframe.js

@@ -154,7 +154,7 @@ Wysiwyg.include({
                 self.$iframe[0].contentWindow.document
                     .open("text/html", "replace")
                     .write(`<!DOCTYPE html><html${
-                        self.options.iframeHtmlClass ? ` class="${self.options.iframeHtmlClass}"` : ''
+                        self.options.iframeHtmlClass ? ' class="' + self.options.iframeHtmlClass +'"' : ''
                     }>${iframeContent}</html>`);
             });
             self.options.document = self.$iframe[0].contentWindow.document;