[FIX] website_slides: correctly upload PDF in course

With website_slides installed go to website, select a course in which
the user is enrolled, try to upload a pdf

Traceback will occur because the category recordset is needed but the id
is passed to _resequence_slides

opw-2229756

closes odoo/odoo#49078

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