Fix another instance of "created at"

Replace it with "created on".

Change-Id: Ibd26bf82d2e64f142a4f3be6da7701e5b035602f

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

@@ -2,7 +2,7 @@
   Deploy key:
   = @key.title
   %small
-    created at
+    created on
     = @key.created_at.stamp("Aug 21, 2011")
 .back-link
   = link_to project_deploy_keys_path(@project) do