UI string: fix message (calendar)

Sorry for breaking the string freeze, but this is quite visible.
The new string landed as part of bugfix: please don't consider
bugfixes as automatically grants for new strings, especially
during the freeze, without *at least* a notification to the
i18n mailing list.

CCMAIL: kde-i18n-doc@kde.org
CCMAIL: sknauss@kde.org