Add 'n' to list of possible values to pg_default_acl.defaclobjtype

This was missed in commit ab89e465; backpatch to v10.

Author: Fabien Coelho <coelho@cri.ensmp.fr>
Discussion: https://postgr.es/m/alpine.DEB.2.21.1807302243001.13230@lancre

doc/src/sgml/catalogs.sgml

@@ -2783,7 +2783,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</>:<replaceable>&lt;salt&gt;<
        <literal>r</> = relation (table, view),
        <literal>S</> = sequence,
        <literal>f</> = function,
-       <literal>T</> = type
+       <literal>T</> = type,
+       <literal>n</> = schema
       </entry>
      </row>