Change path in example of file_fdw for logs

It's better to use a relative path into the data directory, than to a
hardcoded home directory of user 'josh'.

Discussion: https://postgr.es/m/CABUevEyuf67Yu_r9gpDMs5MKifK7+-+pe=ZjKzya4JEn9kUk1w@mail.gmail.com

doc/src/sgml/file-fdw.sgml

@@ -267,7 +267,7 @@ CREATE FOREIGN TABLE pglog (
   application_name text,
   backend_type text
 ) SERVER pglog
-OPTIONS ( filename '/home/josh/data/log/pglog.csv', format 'csv' );
+OPTIONS ( filename log/pglog.csv', format 'csv' );
 </programlisting>
   </para>