Fix wrong default algorithm in openssl pkcs12 help

The default that pkcs12 -export uses is SHA256 and not SHA1.

CLA: Trivial

Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18904)