[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.