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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/459576303dfb47d5b9626331df1f8e8767840ede

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