Well I follow the section 4.2, and correct a wrong path in my slapd.conf, oups !
So now I am able to launch slapd. I test If it works with the command:
openssl s_client -connect localhost:636 -showcerts and before I change the flag in rc.conf:
slapd_flags=' -h "ldapi://%2fvar%2frun%2fopenldap%2fldapi/ ldap://0.0.0.0/ ldaps:///"'
and I can see the certificate. So it seems good. Thanks for all.
François