docs: Link to the correct protocol version inspection function

 docs: Link to the correct protocol version inspection function
 
 The docs for max_protocol_version suggested PQprotocolVersion()
 instead of PQfullProtocolVersion() to find out the exact protocol
 version.  Since PQprotocolVersion() only returns the major protocol
 version, that is bad advice.
 
 Author: Jelte Fennema-Nio <postgres@jeltef.nl>
 Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
 Discussion: https://www.postgresql.org/message-id/flat/CAGECzQSKFxQsYAgr11PhdOr-RtPZEdAXZnHx6U3avLuk3xQaTQ%40mail.gmail.com