Update UK Early May bank holiday for 2020

Summary:
Reference: [[ https://www.gov.uk/bank-holidays | UK bank holidays - GOV.UK ]]
Special thanks: Ian Neal, who reported the bug and suggested the change

BUG: 409189

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: iann, dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22099