doc: fix description of mac "block-size" parameter

doc: fix description of mac "block-size" parameter

The macro for "block-size" is OSSL_MAC_PARAM_BLOCK_SIZE, and this
parameter is not settable.  Refer to the "customization string" rather
than the "custom value" (in the Blake2 spec, this is called the
personalization string).

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22763)