fix BookmarkValueType

[saitoukun]

Thanks for the cool package!

Fixes #18

---

This is how it looks in notion.

(Sorry for the Japanese words mixed in with the examples.)

The value when notion fails to fetch the bookmark.

{
  id: '{your_id}',
  version: 3,
  type: 'bookmark',
  properties: { link: [ [Array] ] },
  created_time: 1593341091859,
  last_edited_time: 1593341040000,
  parent_id: {your_id}',
  parent_table: 'block',
  alive: true,
  copied_from: '{your_id}'
  created_by_table: 'notion_user',
  created_by_id: '{your_id}'
  last_edited_by_table: 'notion_user',
  last_edited_by_id: '{your_id}'
  shard_id:'{your_id}'
  space_id: '{your_id}'
}

---

If you accept this PR, you will see this.

[tobiaslins]

Thank you! Looking good :)