Merge pull request #19004 from danieldevries/fix-migration-fieldname-in-example

fix column name in migration example [ci skip]

activerecord/lib/active_record/migration.rb

@@ -168,7 +168,7 @@ def initialize
   # This will generate the file <tt>timestamp_add_fieldname_to_tablename</tt>, which will look like this:
   #   class AddFieldnameToTablename < ActiveRecord::Migration
   #     def change
-  #       add_column :tablenames, :field, :string
+  #       add_column :tablenames, :fieldname, :string
   #     end
   #   end
   #