Fix inaccurate comment about default nonce length in demos/cipher/aesccm.c

Fixes #25270

CLA: trivial

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25318)

(cherry picked from commit f2b7a00f)