[applets/digital clock] Unbreak the time zone config dialog

a699acd3 introduced a syntax error,
which prevented the time zone config dialog of the digital clock applet
from being shown:

"file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/
    contents/ui/configTimeZones.qml:144:38: Expected token `:'"

I assume the intention was to only show the check box if there are
multiple timezones configured, so "visible" should be the forgotten
property.

(cherry picked from commit 8f999aff0bd44f32acc9921119ce4270bd71399c)