Correct date label on project information page

The label "Created at" would be appropriate for a timestamp such
as "09:30".

However the timestamp is actually being displayed as a date in
the format "Oct 15, 2013".  In this case "Created on" is better.

app/views/admin/projects/show.html.haml

@@ -35,7 +35,7 @@
             = @project.creator.try(:name) || '(deleted)'
         %li
-          %span.light Created at:
+          %span.light Created on:
           %strong
             = @project.created_at.stamp("March 1, 1999")