doc: fix overriding of prefix option

Make the example in the "Building a debug build" section work as
intended.

Fixes: https://github.com/nodejs/node/issues/30477

PR-URL: https://github.com/nodejs/node/pull/30518


Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>