Fix up references to scram-sha-256

pg_hba_file_rules erroneously reported this as scram-sha256.  Fix that.

To avoid future errors and confusion, also adjust documentation links
and internal symbols to have a separator between "sha" and "256".

Reported-by: Christophe Courtois <christophe.courtois@dalibo.com>
Author: Michael Paquier <michael.paquier@gmail.com>