[FIX] doc: xpath invalid for 8.0

The current xpath was the one for saas-6 or future 9.0 but didn't work
in version 8.

closes #7715
opw-645555

note: this commit should not be forward ported after 8.0

doc/howtos/themes.rst

@@ -619,7 +619,7 @@ 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_structure']//div[@class='o_panel_body']" position="inside">
+     <xpath expr="//div[@id='snippet_structure']" position="inside">
        <t t-snippet="theme_tutorial.snippet_testimonial"
           t-thumbnail="/theme_tutorial/static/src/img/ui/snippet_thumb.jpg"/>
      </xpath>