Fix import in pkey docs

//! extern crate openssl;

//!

//! use openssl::rsa::Rsa;

//! use openssl::pkey::Pkey;

//! use openssl::pkey::PKey;

//!

//! fn main() {

//!     let rsa = Rsa::generate(2048).unwrap();

//!     let pkey = Pkey::from_rsa(rsa).unwrap();

//!     let pkey = PKey::from_rsa(rsa).unwrap();

//! }

//! ```