[FIX] website_mail_group: restore missing snippet icon

The file was originally added in website_mail at 59b934
in saas-4 but was lost in master since.

addons/website_mail_group/views/snippets.xml

@@ -7,7 +7,7 @@
     <xpath expr="//div[@id='snippet_content']" position="inside">
         <div>
             <div class="oe_snippet_thumbnail">
-                <img class="oe_snippet_thumbnail_img" src="/website_mail/static/src/img/blocks/button_group_subscribe.png"/>
+                <img class="oe_snippet_thumbnail_img" src="/website_mail_group/static/src/img/blocks/button_group_subscribe.png"/>
                 <span class="oe_snippet_thumbnail_title">Discussion Group</span>
             </div>