doc: fix "sign.verify" typo in crypto doc.

Fix typo in example

PR-URL: https://github.com/nodejs/node/pull/7411


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>