max_prepared_foreign_transactions is unrecognized

[Eric J. Van der Velden]

On https://wiki.postgresql.org/wiki/2PC_on_FDW it says I have to set the
configuration parameter "max_prepared_foreign_transactions"  in
postgresql.conf. When I restart postgres, I see:

LOG:  unrecognized configuration parameter
"max_prepared_foreign_transactions" in file
"/var/lib/postgresql/data/postgresql.conf" line 689

Thanks,
Eric.