update docs to include arc4 example
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 9d18ce5..77e61b5 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -159,6 +159,17 @@
``192``, or ``256`` bits in length. This must be kept
secret.
+ .. doctest::
+
+ >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+ >>> algorithm = algorithms.ARC4(key)
+ >>> cipher = Cipher(algorithm, mode=None)
+ >>> encryptor = cipher.encryptor()
+ >>> ct = encryptor.update(b"a secret message")
+ >>> decryptor = cipher.decryptor()
+ >>> decryptor.update(ct)
+ 'a secret message'
+
.. _symmetric-encryption-modes: