[FIX] website_slides: fixed the traceback when add a new section on a course

in this commit, we have set the correct attribute(field's name) to fix
the traceback while we adding a new section on a course using website

task-2053822

closes odoo/odoo#35786

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>