Fixes simple error in _validate_clusterer

[pujaltes]

Reference issues/PRs

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

Screenshots (if appropriate)

Any other comments?

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed. I used pytest to check this on Python tests.

[matteocao]

thank you for your contribution @pujaltes !

I will force merge this PR and try to make sure the CI runs again.