Bug 27280: (bug 27026 follow-up) Make consistent the explanation for "Days mode"

Bug 27280: (bug 27026 follow-up) Make consistent the explanation for "Days mode"

We have "Skip closed days" in the dropdown, but "Calendar" on the table. Both must match

Test plan:
Go to the circ rules, select a value for "Days mode", save the rule,
confirm that the dropdown list values are matching the ones we displayed
in the cells

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>