[FIX] calendar: correct description of 'free' option

On a calendar event, the 'free' option does not hide the element.
So that the user doesn't expect this behavior,
you have to modify its description.

opw-3047999

closes odoo/odoo#117599

X-original-commit: ecc850f4
Signed-off-by: Arnaud Joset <arj@odoo.com>
Signed-off-by: Lefebvre Thomas (thle) <thle@odoo.com>