Incorrect link on SSL certificate creation page

[willmm]

The SSL certificate creation page (https://doc.dovecot.org/2.3/admin_manual/ssl/certificate_creation/) points to the mkcert.sh file on the tools page. It's actually at https://github.com/dovecot/core/blob/main/doc/mkcert.sh.

[cmouse]

Probably should point to release-2.3 too...