Fixes a missing import in field prop example in docs/beta/admin/fields.mdx. <!-- For external contributors, please include: - A summary of the pull request and any related issues it fixes. - Reasoning for the changes made or any additional context that may be useful. Ensure you have read and understand the [CONTRIBUTING.md](https://github.com/payloadcms/payload/blob/main/CONTRIBUTING.md) document in this repository. -->