[FIX] google_calendar: fixing request for exclusion

Activate Google Synchronization, create on OE a recurrent event (no allday)
synchronize the calendar, then delete an event of the recursion on OE,
sync again on OE.

An error will popup in the console. There is a typo in the variable
sent to the google API.
https://developers.google.com/calendar/v3/reference/events



closes odoo/odoo#40595

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>