vm: change ContextifyScript to Script in comment

Reading the comment at the top of the vm.js, I think that
ContextifyScript should perhaps just be Script.

PR-URL: https://github.com/nodejs/node/pull/8415


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>