doc: remove unnecessary comma in postgres-fdw

Backpatch-through: master

Author: bmomjian

doc/src/sgml/postgres-fdw.sgml

@@ -124,7 +124,7 @@
      <listitem>
       <para>
        <literal>user</literal>, <literal>password</literal> and <literal>sslpassword</literal> (specify these
-       in a user mapping, instead, or use a service file)
+       in a user mapping instead, or use a service file)
       </para>
      </listitem>
      <listitem>