doc: fence command examples in release guide

Put the command examples for finding and replacing the `REPLACEME`
tags into code fences so that they are more easily copiable when
viewing the guide in the GitHub web UI.

PR-URL: https://github.com/nodejs/node/pull/39544


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Danielle Adams <adamzdanielle@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>