[FIX] website_blog: remove stars from blog description

Current behavior:
When adding a "Heading" in the first 200 characters of a blog with
/Heading, the short description of the blog preview would have
unwanted stars (*) around the heading

Steps to reproduce:
- Create a blog article
- In the first 200 characters of the blog use a heading (e.g. /Heading1)
- Go back to the blog list, the description of the new article contains
  unwanted stars (*)

opw-2798595

closes odoo/odoo#88082

Signed-off-by: Romain Derie (rde) <rde@odoo.com>