Documentation typo fix in BN_bn2bin.pod

Change the description for BN_hex2bn() so that it uses the same BIGNUM argument name as its prototype.

CLA: trivial

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6712)