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

doc/man3/BN_bn2bin.pod

@@ -55,8 +55,8 @@ freed later using OPENSSL_free().
 BN_hex2bn() takes as many characters as possible from the string B<str>,
 including the leading character '-' which means negative, to form a valid
 hexadecimal number representation and converts them to a B<BIGNUM> and
-stores it in **B<bn>. If *B<bn> is NULL, a new B<BIGNUM> is created. If
-B<bn> is NULL, it only computes the length of valid representation.
+stores it in **B<a>. If *B<a> is NULL, a new B<BIGNUM> is created. If
+B<a> is NULL, it only computes the length of valid representation.
 A "negative zero" is converted to zero.
 BN_dec2bn() is the same using the decimal system.