doc: add missing version to update cmd

Add to the example of running `./tools/update-npm.sh` the missing
reference to the version number that is required by the command.

PR-URL: https://github.com/nodejs/node/pull/36204


Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>