message: improved example in Passphrase-based encryption
diff: //! let plaintext = b"Hello world!";
  //! let passphrase = "this is not a good passphrase";
  //!
  +//! // Encrypt the plaintext to a ciphertext using the passphrase...
  +//! let encrypted = {
  //! let encryptor = age::Encryptor::Passphrase(Secret::new(passphrase.to_owned()));
  +//!
  //! let mut encrypted = vec![];
  -//! {
  //! let mut writer = encryptor.wrap_output(&mut encrypted, false)?;
  //! writer.write_all(plaintext)?;
  //! writer.finish()?;
  +//!
  +//! encrypted
  //! };
  //!
  +//! // ... and decrypt the ciphertext to the plaintext again using the same passphrase.
  +//! let decrypted = {
  //! let decryptor = age::Decryptor::Passphrase(Secret::new(passphrase.to_owned()));
  -//! let mut reader = decryptor.trial_decrypt(&encrypted[..], |_| None)?;
  +//!
  //! let mut decrypted = vec![];
  +//! let mut reader = decryptor.trial_decrypt(&encrypted[..], |_| None)?;
  //! reader.read_to_end(&mut decrypted);
  //!
  +//! decrypted
  +//! };
  +//!
  //! assert_eq!(decrypted, plaintext);
  //! # Ok(())
  //! # }
  