Adjust documentation for key certificate retrieval
In https://gitlab.archlinux.org/archlinux/signstar/-/blob/1b792ec6469674a8e6a4028316706a8f947844aa/nethsm/src/lib.rs?page=4#L3401 we define the returned output format as PKCS#8. This is not correct though, as PKCS#8 is used for private key material and we are returning x.509, PEM encoded public keys (correctly documented in nethsm-cli already).