fix typo in old migration

db/migrate/20171109092839_create_join_table_group_offer_categories_volunteers.rb

@@ -3,7 +3,7 @@ class CreateJoinTableGroupOfferCategoriesVolunteers < ActiveRecord::Migration[5.
     create_join_table :group_offer_categories, :volunteers do |t|
       t.index [:group_offer_category_id, :volunteer_id], name: 'index_group_offer_on_volunteer'
       t.datetime :deleted_at, index: true
-      t.timestamp
+      t.timestamps
     end
   end
 end