Remove remaining references to version-0 calling convention in docs.

Support for version-0 calling convention was removed in PostgreSQL v10.
Change the SPI example to use version 1 convention, so that it actually
works.

Author: John Naylor
Discussion: https://www.postgresql.org/message-id/CAJVSVGVydmhLBdm80Rw3G8Oq5TnA7eCxUv065yoZfNfLbF1tzA@mail.gmail.com