transitOptionsPanel: Fix one-off error when setting date

When setting the GtkCalendar date when clicking the date
button when using "Leave By" or "Arrive By" we need to
subtact one to the month value, as the select_month()
method expects 0-based months.
1 job for wip/mlundblad/fix-transit-calender-month-one-off in 7 minutes and 13 seconds (queued for 1 second)