doc: remove invalid padding from privateEncrypt

PR-URL: https://github.com/nodejs/node/pull/9611
Fixes: https://github.com/nodejs/node/issues/9609


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Sam Roberts <sam@strongloop.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>