typo in pki_key_generate_ecdsa function

[Yury Korolev]

Hi,

There is a typo in pki_key_generate_ecdsa function switch case 521 value. It 
should be 521. 

https://gitlab.com/libssh/libssh-mirror/blob/master/src/pki_crypto.c#L561 
<https://gitlab.com/libssh/libssh-mirror/blob/master/src/pki_crypto.c#L561>