docs: add backend_type to file-fdw CSV log example

backend_type was added to the CVS log output in commit 70a7b47.

Reported-by: Fabrízio de Royes Mello

Discussion: https://postgr.es/m/CAFcNs+ruvRks3BV1j7yQ-MvxsswmKJa0cVh2yK5Dd-xXVM8wPw@mail.gmail.com

Backpatch-through: master

Author: bmomjian

doc/src/sgml/file-fdw.sgml

@@ -261,7 +261,8 @@ CREATE FOREIGN TABLE pglog (
   query text,
   query_pos integer,
   location text,
-  application_name text
+  application_name text,
+  backend_type text
 ) SERVER pglog
 OPTIONS ( filename '/home/josh/data/log/pglog.csv', format 'csv' );
 </programlisting>