doc: Fix incorrect openssl option

doc: Fix incorrect openssl option

The openssl command for displaying the DN of a client certificate was
using --subject and not the single-dash option -subject. While recent
versions of openssl handles double dash options,  earlier does not so
fix by using just -subject  (which is per the openssl documentation).

Backpatch to v14 where this was introduced.

Reported-by: konkove@gmail.com
Discussion: https://postgr.es/m/170672168899.666.10442618407194498217@wrigleys.postgresql.org
Backpatch-through: v14