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

doc/src/sgml/libpq.sgml

@@ -2220,7 +2220,7 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
         server does not support the protocol version requested by the client,
         the connection is automatically downgraded to a lower minor protocol
         version that the server supports. After the connection attempt has
-        completed you can use <xref linkend="libpq-PQprotocolVersion"/> to
+        completed you can use <xref linkend="libpq-PQfullProtocolVersion"/> to
         find out which exact protocol version was negotiated.
        </para>