x11-themes/kde-icons-black-and-white: change WWW to less dead site

The old www.kde-look.org is dead, kde-look.org redirects
to store.kde.org. Change WWW to the latter.

PR: 268419
Reported by: grahamperrin
Approved by: eduardo (mentor)
Differential Revision: https://reviews.freebsd.org/D38521