remove_column does not take a type argument. [ci skip]. Closes rails#12864

Author: senny

guides/source/migrations.md

@@ -179,7 +179,7 @@ generates
 ```ruby
 class RemovePartNumberFromProducts < ActiveRecord::Migration
   def change
-    remove_column :products, :part_number, :string
+    remove_column :products, :part_number
   end
 end
 ```