Remove s to Bulles in content_title

When we add or edit a Bulle, there's an "s" in the title: Bulles instead of Bulle.

I don't know where to find it, so I leave it here.
It's really not important but during a demo yesterday it hurt my eyes ;)

Start by searching the backend for the content_title label or the displayed “Bulles” text, then trace the create/edit Bulle entry point. The work is done when both adding and editing a Bulle display “Bulle” without the extra “s”; verify both flows in the relevant tests or backoffice view.

Written by the indexing model from the issue text.