Fixes #4459 "issuserAltName" documentation typo.

See crypto/objects/objects.txt:767 -- field is "issuerAltName"
CLA: trivial

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4460)