Correct padding mode flag name for EVP_PKEY_decrypt/encrypt() examples

The example code in EVP_PKEY_decrypt(3) and EVP_PKEY_encrypt(3) and uses
a nonexistent padding mode `RSA_OAEP_PADDING`, which should be
`RSA_PKCS1_OAEP_PADDING` instead.

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18255)