Fix Workspace KCM translation domain name

Summary:
TRANSLATION_DOMAIN and value in Messages.sh need to be the same.
Use the new name kcm5_workspace for consistency.

Reviewers: Plasma, ltoscano, yurchor

Reviewed By: ltoscano

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12956