Better name for group tab

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

app/views/layouts/nav/_group.html.haml

@@ -11,7 +11,7 @@
     = link_to group_path(@group), title: 'Home', data: {placement: 'right'} do
       = icon('dashboard fw')
       %span
-        Activity
+        Group
   - if current_user
     = nav_link(controller: [:group, :milestones]) do
       = link_to group_milestones_path(@group), title: 'Milestones', data: {placement: 'right'} do