[FIX] website_slides: fix slide.slide statistics compute method triggers

The slide.slide "_compute_slides_statistics" method is missing the 'slide_ids.sequence' trigger.
It should be specified for cases when one of the slide_ids changes category (determined by the
sequence).

This can cause some CacheMiss and seem related to nondeterministic errors on the runbot.

PR #40705
Task#2123711
Runbot build error ID 247

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