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

doc/src/sgml/client-auth.sgml

@@ -666,7 +666,7 @@ include_dir         <replaceable>directory</replaceable>
        format. To see the <literal>DN</literal> of a client certificate
        in this format, do
 <programlisting>
-openssl x509 -in myclient.crt -noout --subject -nameopt RFC2253 | sed "s/^subject=//"
+openssl x509 -in myclient.crt -noout -subject -nameopt RFC2253 | sed "s/^subject=//"
 </programlisting>
         Care needs to be taken when using this option, especially when using
         regular expression matching against the <literal>DN</literal>.