doc: fix misleading language in vm docs

The note following the http.Server example in the vm documentation
contains misleading language. This commit removes the incorrect
reference to threads.

Fixes: https://github.com/nodejs/node/issues/10697
PR-URL: https://github.com/nodejs/node/pull/10708


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>