Use nacl.secret.SecretBox.NONCE_SIZE instead of a hardcoded 24

docs/secret.rst

@@ -31,7 +31,7 @@ Example
     # This is a nonce, it *MUST* only be used once, but it is not considered
     #   secret and can be transmitted or stored alongside the ciphertext. A
     #   good source of nonce is just 24 random bytes.
-    nonce = nacl.random(24)
+    nonce = nacl.random(nacl.secret.SecretBox.NONCE_SIZE)
 
     # Encrypt our message, it will be exactly 16 bytes longer than the original
     #   message as it stores authentication information alongside it.