tools: simplify tools/doc/preprocess.js

PR-URL: https://github.com/nodejs/node/pull/19539


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>