[FIX] calendar : change stop date for recurrent event

Changing end time for recurrent event will generate an error.

steps to reproduce the error:
1- Create an event in the calendar app with recurrence on
2- Save and close
3- Edit this event again and select change all events at the top
4- Change the ending time

The error was happening because the wrong key was accessed in a
dictionary

opw-3236432

closes odoo/odoo#117458

Signed-off-by: Adrien Widart <awt@odoo.com>