[FIX] doc: recommand to xpath on snippet_structure

Fixed the xpath expression for inserting snippet into Structure tab

Closes #22971

doc/howtos/themes.rst

@@ -614,7 +614,7 @@ The previous code will create the snippet’s content, but we still need to plac
 .. code-block:: xml
 
    <template id="place_into_bar" inherit_id="website.snippets" name="Place into bar">
-     <xpath expr="//div[@id='snippet_content']/div[@class='o_panel_body']" position="inside">
+     <xpath expr="//div[@id='snippet_structure']/div[@class='o_panel_body']" position="inside">
        <t t-snippet="theme_tutorial.snippet_testimonial"
           t-thumbnail="/theme_tutorial/static/src/img/ui/snippet_thumb.jpg"/>
      </xpath>