doc: add missing code formatting in vm.md

This commit adds missing code formatting in a few places in
the vm module documentation.

PR-URL: https://github.com/nodejs/node/pull/31350


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>