Fix formatting for PersistField

[parsonsmatt]

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock
• Ran fourmolu on any changed files (restyled will do this for you, so
  accept the suggested changes if it makes them)
• Adhered to the code style (see the .editorconfig and fourmolu.yaml files for details)

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Bumped the version number if there isn't an (unreleased) on the Changelog
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)