transitOptionsPanel: Fix one-off error when setting date

Merged Marcus Lundblad requested to merge wip/mlundblad/fix-transit-calender-month-one-off into main

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.

Fixes #447 (closed)

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