fix: Typo in the notification invitation message (#6346)

Author: smcri

app/api/helpers/system_notifications.py

@@ -392,7 +392,7 @@ def get_invite_papers_notification_actions(cfs_link, submit_link):
     },
     EVENT_ROLE: {
         'title': u'Invitation to be {role_name} at {event_name}',
-        'message': u"You've been invited to be a <strong>{role_name}</strong>" +
+        'message': u"You've been invited to be one of the <strong>{role_name}s</strong>" +
                    u" at <strong>{event_name}</strong>.",
         'recipient': 'User',
     },