erp5_system_event: Assignee can also trigger acknowledge transition.

bt5/erp5_system_event/WorkflowTemplateItem/portal_workflow/system_event_workflow/transition_acknowledge.xml

@@ -50,6 +50,7 @@
             <value>
               <tuple>
                 <string>Assignor</string>
+                <string>Assignee</string>
                 <string>Manager</string>
               </tuple>
             </value>