bug 4900: add in missing value for usedaysmode pref

The usedaysmode preference was missing the option to
calculate due date by finding the next open date on the
calendar instead of ignoring all days the library is
closed.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>

koha-tmpl/intranet-tmpl/prog/en/modules/admin/preferences/circulation.pref

@@ -140,11 +140,12 @@ Circulation:
               class: date
             - .
         -
+            - Calculate the due date using 
             - pref: useDaysMode
               choices:
-                  Days: Include
-                  Calendar: "Don't include"
-            - days when the library in closed when calculating due dates.
+                  Days: circulation rules only.
+                  Calendar: the calendar to skip all days the library is closed.
+                  Datedue: the calendar to push the due date to the next open day
         -
             - When renewing checkouts, base the new due date on
             - pref: RenewalPeriodBase