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

doc/man3/EVP_PKEY_CTX_set_hkdf_md.pod

@@ -139,7 +139,7 @@ salt value "salt" and info value "label":
      /* Error */
  if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
      /* Error */
- if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 0)
+ if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
      /* Error */
  if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
      /* Error */