demos/mac/cmac-aes256: Clarify the cipher algorithm used
demos/mac/cmac-aes256: Clarify the cipher algorithm used

The currently used cipher is aes256 which is an alias to AES-256-CBC,
so the demo is correct.
However it might be misleading so make it clear the CBC mode
cipher is used.

Reviewed-by: Dmitry Belyavskiy <38c64d6e24766247aad56af3a6ddb0056ad36a44@gmail.com>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19401)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl