doc: fix link to cli.md in vm.md

Looks like the link to cli.md is missing in vm.md:
https://nodejs.org/api/vm.html#vm_vm_runindebugcontext_code

Added the link which can be verified by using the following commands:
$ make doc
$ open out/doc/api/vm.html

PR-URL: https://github.com/nodejs/node/pull/9481


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>