fix typo in activity and entities

lib/api/entities.rb

@@ -20,7 +20,7 @@ module API
 
     class UserActivity < Grape::Entity
       expose :username
-      expose :date
+      expose :last_activity_at
     end
 
     class Identity < Grape::Entity

lib/gitlab/user/activity.rb

@@ -8,7 +8,7 @@ module Gitlab
         @time = time
       end
 
-      def lat_activity_at
+      def last_activity_at
         @lat_activity_at ||= Time.at(@time).to_s(:db)
       end
     end