Fix typo in HKDF example documentation

Out-of-bounds array access in the example documentation of
EVP_PKEY_CTX_set_hkdf_md fixed.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4081)