Doc: Fix incorrect reference to conflicting column in pg_replication_slots.

Commit 007693f2a3 changes the existing 'conflicting' field to
'conflict_reason' in pg_replication_slots but missed updating one of its
existing references.

Author: Hou Zhijie
Discussion: https://postgr.es/m/OS0PR01MB571690299199ACA80F602D97947E2@OS0PR01MB5716.jpnprd01.prod.outlook.com

doc/src/sgml/ref/pgupgrade.sgml

@@ -440,8 +440,8 @@ make prefix=/usr/local/pgsql.new install
       <para>
        All slots on the old cluster must be usable, i.e., there are no slots
        whose
-       <link linkend="view-pg-replication-slots">pg_replication_slots</link>.<structfield>conflicting</structfield>
-       is <literal>true</literal>.
+       <link linkend="view-pg-replication-slots">pg_replication_slots</link>.<structfield>conflict_reason</structfield>
+       is not <literal>NULL</literal>.
       </para>
      </listitem>
      <listitem>