Doc: fix minor typo in func.sgml.

Discussion: https://postgr.es/m/20191119.222048.49467220816510881.t-ishii%40sraoss.co.jp

doc/src/sgml/func.sgml

@@ -20927,7 +20927,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
         (<parameter>slot_name</parameter> <type>name</type>, <parameter>lsn</parameter> <type>pg_lsn</type>)
        </entry>
        <entry>
-        Copies an existing logical replication slot name <parameter>src_slot_name</parameter>
+        Copies an existing logical replication slot named <parameter>src_slot_name</parameter>
         to a logical replication slot named <parameter>dst_slot_name</parameter>
         while changing the output plugin and persistence. The copied logical slot starts
         from the same <acronym>LSN</acronym> as the source logical slot. Both