[FIX] dev/howtos/themes: correct wrong code example

Since the update of the themes tutorial, the id of the snippet template was modified
but this part of the tutorial wasn't updated accordingly.

This commit updates the referenced template id to make sure the tutorial works
as expected.

Finetuning of https://github.com/odoo/odoo/commit/08d44d444490713ca39618a67f441bbd84b8b900

content/developer/howtos/themes.rst

@@ -845,7 +845,7 @@ We start by adding a new file in our views folder - name it **options.xml** and
 
     <template id="snippet_testimonial_opt" name="Snippet Testimonial Options" inherit_id="website.snippet_options">
         <xpath expr="//div[@data-js='Box']" position="after">
-            <div data-selector=".snippet_testimonial"> <!-- Options group -->
+            <div data-selector=".s_testimonial"> <!-- Options group -->
                 <div class="dropdown-submenu">
                     <a href="#" class="dropdown-item">Your Option</a>
                     <div class="dropdown-menu"><!-- Options list -->