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 <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19401)

demos/mac/cmac-aes256.c

@@ -72,7 +72,7 @@ int main(void)
     unsigned char *out = NULL;
     size_t out_len = 0;
     OSSL_PARAM params[4], *p = params;
-    char cipher_name[] = "aes256";
+    char cipher_name[] = "AES-256-CBC";
 
     library_context = OSSL_LIB_CTX_new();
     if (library_context == NULL) {