Use Padding constant in RSA docs example

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

//!     let data: Vec<u8> = String::from("foobar").into_bytes();

//!     let mut encrypted_data: Vec<u8>  = vec![0; 512];

//!     let padding = Padding::from_raw(1); // PKCS1 Padding

//!     let padding = Padding::PKCS1;

//!     let _ = rsa.public_encrypt(&data, encrypted_data.as_mut_slice(), padding).unwrap();

//! }

//! ```