doc: Use system-username instead of system-user

This inconsistency has been introduced in efb6f4a4f9.

Reported-by: Julien Rouhaud
Author: Bertrand Drouvot
Discussion: https://postgr.es/m/ZdMWux1HpIebkEmd@ip-10-97-1-34.eu-west-3.compute.internal
Backpatch-through: 16

doc/src/sgml/client-auth.sgml

@@ -965,8 +965,8 @@ local   db1,db2,@demodbs  all                                   md5
    external authentication system with the database user name that the
    user has requested to connect as. The value <literal>all</literal>
    can be used as the <replaceable>database-username</replaceable> to specify
-   that if the <replaceable>system-user</replaceable> matches, then this user
-   is allowed to log in as any of the existing database users. Quoting
+   that if the <replaceable>system-username</replaceable> matches, then this
+   user is allowed to log in as any of the existing database users. Quoting
    <literal>all</literal> makes the keyword lose its special meaning.
   </para>
   <para>