pgsql: pgcrypto: Tweak error message for incorrect session key length

pgcrypto: Tweak error message for incorrect session key length

The error message added in 379695d3cc70 referred to the public key being
too long. This is confusing as it is in fact the session key included
in a PGP message which is too long. This is harmless, but let's be
precise about what is wrong.

Per offline report.

Reported-by: Zsolt Parragi <zsolt.parragi@percona.com>
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/770d13dd063cb5fb89953a0ffb68b70be716ffa6

Modified Files
--------------
contrib/pgcrypto/expected/pgp-pubkey-session.out | 2 +-
contrib/pgcrypto/px.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)