Correct return type of `Generator::unique()`

[DannyvdSluijs]

What is the reason for this PR?

Generator::unique() is documented as returning a Generator where it is actually returning a UniqueGenerator as annotated on the class property. This PR corrects this minor incorrection. Going through the issues and PR I didn't find anything related to this correct other that the initial commit that introduced the function.

• [ ] A new feature
• [X] Fixed an issue

Author's checklist

• [X] Follow the Contribution Guide
• [X] New features and changes are documented

Summary of changes

This PR corrects the return type in the PHPDOC block of the Generator::unique() function

Review checklist

• [ ] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [ ] Changes are approved by maintainer

[localheinz]

Thank you, @DannyvdSluijs!