Fix ActiveRecord::Callbacks sample code [ci skip]

Callback caller class uses `after_initialize`,
but Callback callee defines `after_find`.

Current sample code causes following error.

  NoMethodError: undefined method `after_initialize' for #<EncryptionWrapper:0x007fe4931fa5c0>
Merge pull request #13143 from joker1007/fix_active_record_callbacks_document_bug

Fix ActiveRecord::Callbacks sample code [ci skip]