message: Fix some errors in HOWTO and make it easier for copy&paste.
diff: @@ -159,6 +159,9 @@ encrypt or decrypt data:
  import tink
  from tink import aead
  +plaintext = b'your data...'
  +associated_data = b'context'
  +
  # Register all AEAD primitives
  aead.register()
  @@ -169,7 +172,7 @@ encrypt or decrypt data:
  aead_primitive = keyset_handle.primitive(aead.Aead)
  # 3. Use the primitive.
  - ciphertext = aead_primitive.encrypt(plaintext, associated data)
  +ciphertext = aead_primitive.encrypt(plaintext, associated_data)
  ```
  ### Deterministic symmetric key encryption
  @@ -182,6 +185,9 @@ primitive to encrypt or decrypt data:
  import tink
  from tink import daead
  +plaintext = b'your data...'
  +associated_data = b'context'
  +
  # Register all deterministic AEAD primitives
  daead.register()
  @@ -192,7 +198,7 @@ primitive to encrypt or decrypt data:
  daead_primitive = keyset_handle.primitive(daead.DeterministicAead)
  # 3. Use the primitive.
  - ciphertext = daead_primitive.encrypt_deterministically(plaintext, associated data)
  +ciphertext = daead_primitive.encrypt_deterministically(plaintext, associated_data)
  ```
  ### Message Authentication Code
  @@ -204,6 +210,8 @@ You can compute or verify a
  import tink
  from tink import mac
  +data = b'your data...'
  +
  # Register all MAC primitives
  mac.register()
  @@ -230,6 +238,9 @@ one can use the following:
  import tink
  from tink import hybrid
  +plaintext = b'your data...'
  +context = b'context'
  +
  # Register all Hybrid primitives
  hybrid.register()
  @@ -254,7 +265,6 @@ one can use the following:
  # 3. Use the primitive.
  plaintext = hybrid_decrypt.decrypt(ciphertext, context)
  -
  ```
  ### Digital Signatures
  @@ -277,7 +287,7 @@ You can sign or verify using a
  signer = keyset_handle.primitive(signature.PublicKeySign)
  # 3. Use the primitive to sign.
  - signature = signer.sign(b'your data')
  +signature_data = signer.sign(b'your data')
  # Verifying
  # 1. Obtain the public key material.
  @@ -287,7 +297,7 @@ You can sign or verify using a
  verifier = public_keyset_handle.primitive(signature.PublicKeyVerify)
  # 3. Use the primitive to verify.
  - verifier.verify(signature, b'your data')
  +verifier.verify(signature_data, b'your data')
  ```
  ### Envelope encryption
  @@ -307,6 +317,9 @@ using the credentials in `credentials.json` as follows:
  key_uri = 'gcp-kms://projects/tink-examples/locations/global/keyRings/foo/cryptoKeys/bar'
  gcp_credentials = 'credentials.json'
  +plaintext = b'your data...'
  +associated_data = b'context'
  +
  # Read the GCP credentials and setup client
  try:
  gcp_client = gcpkms.GcpKmsClient(key_uri, gcp_credentials)
  @@ -322,6 +335,7 @@ using the credentials in `credentials.json` as follows:
  except tink.TinkError as e:
  logging.error('Error creating primitive: %s', e)
  return 1
  +
  # Use env_aead to encrypt data
  - ciphertext = env_aead.encrypt(plaintext, associated data)
  +ciphertext = env_aead.encrypt(plaintext, associated_data)
  ```
  