doc: use correct tls certificate property name

Docs referred to an `issuer` property being optionally present, when it
should have referred to the `issuerCertificate` property.

PR-URL: https://github.com/nodejs/node/pull/10389


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>