bin/post_release.py: Add .html to hrefs

oops.

Fixes: 3226b12a
       ("release: Add an update_release_calendar.py script")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 7e4b87f9)