[FIX] website_{}: show correct placeholder in search bar

before this commit, in the mobile view, in the search bar
of the website users(forum) and events leader board,
the placeholder is shown as Search courses

after this commit, the wrong placeholder will be updated
to Search users in the placeholder as in the normal view.

closes odoo/odoo#115131

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