Update example in Migration Style guide

- Remove unnecessary mixing with migration helper methods.
- Passing a hash containing `:from` and `:to` will make this change
reversible in migration.

doc/development/migration_style_guide.md

@@ -424,16 +424,10 @@ Take the following migration as an example:
 
 ```ruby
 class DefaultRequestAccessGroups < ActiveRecord::Migration[5.2]
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
 
-  def up
-    change_column_default :namespaces, :request_access_enabled, true
-  end
-
-  def down
-    change_column_default :namespaces, :request_access_enabled, false
+  def change
+    change_column_default(:namespaces, :request_access_enabled, from: false, to: true)
   end
 end
 ```

generator_templates/active_record/migration/create_table_migration.rb

 # frozen_string_literal: true
 
-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 
 class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]

generator_templates/active_record/migration/migration.rb

 # frozen_string_literal: true
 
-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 
 class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]

generator_templates/rails/post_deployment_migration/migration.rb

 # frozen_string_literal: true
 
-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 
 class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]