Doc: fix typo in "Generic File Access Functions" section.

Issue reported by me and fix by Tom Lane.
Discussion: https://postgr.es/m/20181219.080458.1434575730369741406.t-ishii%40sraoss.co.jp

Author: tatsuo-ishii

doc/src/sgml/func.sgml

@@ -20116,8 +20116,8 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
    </para>
 
    <para>
-    Note that granting users the EXECUTE privilege on the
-    <function>pg_read_file()</function>, or related, functions allows them the
+    Note that granting users the EXECUTE privilege on
+    <function>pg_read_file()</function>, or related functions, allows them the
     ability to read any file on the server which the database can read and
     that those reads bypass all in-database privilege checks.  This means that,
     among other things, a user with this access is able to read the contents of the