FIX: Move misplaced error translation (#574)

The key and the error message are there, but not nested in the right place.

config/locales/server.en.yml

@@ -77,9 +77,10 @@ en:
         models:
           event:
             only_one_event: "A post can only have one event."
-            only_group: "An event accepts only group names."
             must_be_in_first_post: "An event can only be in the first post of a topic."
             raw_invitees_length: "An event is limited to %{count} users/groups."
+            raw_invitees:
+              only_group: "An event accepts only group names."
             ends_at_before_starts_at: "An event can't end before it starts."
             start_must_be_present_and_a_valid_date: "An event requires a valid start date."
             end_must_be_a_valid_date: "End date must be a valid date."