Bump plek from 3.0.0 to 4.0.0
Bump plek from 3.0.0 to 4.0.0

Bumps [plek](https://github.com/alphagov/plek) from 3.0.0 to 4.0.0.
- [Release notes](https://github.com/alphagov/plek/releases)
- [Changelog](https://github.com/alphagov/plek/blob/master/CHANGELOG.md)
- [Commits](https://github.com/alphagov/plek/compare/v3.0.0...v4.0.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
alphagov/service-manual-publisher,alphagov/service-manual-publisher,alphagov/service-manual-publisher