[FIX] gamification: missing model in email template

Therefore, the template could be seen from
the Discuss module, when clicking on "Send Mail",
but, when attempting to use it, it failed
(as it was not applied on the right model).

opw-670546

addons/gamification/data/goal_base.xml

@@ -20,6 +20,7 @@
         
         <record id="simple_report_template" model="mail.template">
             <field name="name">Simple Challenge Report Progress</field>
+            <field name="model_id" ref="gamification.model_gamification_challenge"/>
             <field name="body_html"><![CDATA[
 <header>
     <strong>${object.name}</strong>