Fixed EVP_MAC_final argument count in example

Fixed EVP_MAC_final argument count in example

EVP_MAC_final had only three arguments / the buffer/tag size was missing.
Fixes #12424

Note, that I didn't try to compile the example to look for other problems.

Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12429)