Make the example error free

Without padding b"my secret message" is not divisible by AES's
block size and thus would throw an error.
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index 0812f6b..9986d89 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -14,7 +14,7 @@
 
         >>> from cryptography.primitives.block import BlockCipher, ciphers, modes
         >>> cipher = BlockCipher(ciphers.AES(key), modes.CBC(iv))
-        >>> cipher.encrypt("my secret message") + cipher.finalize()
+        >>> cipher.encrypt(b"a secret message") + cipher.finalize()
         # The ciphertext
         [...]