Replace genrsa with genpkey

[dol]

According to the OpenSSL documentation the genrsa command is superseded by genpkey

E.g.:

openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:4096

[diafygi]

Eh, it's a bit more complex of a command, so I'll stick with genrsa until Let's Encrypt starts accepting elliptic curve public keys.

[dol]

If you copy paste it, it not an argument about the complexity of the command. But it your decision.