doc: use change to modify migrations

book/02_getting_started.md

@@ -241,14 +241,10 @@ upon each other like LEGO blocks. Each new Migrations must have different
 class's names, is a good idea name migration's explicit. For exmaple: 
 
       class AddTagColumn < V 1.1
-        def self.up
+        def self.change
          add_column Page.table_name, :tag, :string
          Page.reset_column_information
         end
-
-        def self.down
-          drop_table Page.table_name
-        end
       end
 
 Now we just need to tell Camping to use our migration. Write this at the bottom of nuts.rb