Fix header title

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/groups/activity.html.haml

@@ -3,7 +3,7 @@
     = auto_discovery_link_tag(:atom, group_url(@group, format: :atom, private_token: current_user.private_token), title: "#{@group.name} activity")
 
 - page_title    "Activity"
-- header_title  "Activity", activity_dashboard_path
+- header_title  group_title(@group, "Activity", activity_group_path(@group))
 
 %section.activities
   = render 'activities'