doc: indent ordered list child content

Markdownlint flags this with MD029 rule.
Markdown renders will usually use list continuation number if it can.
Explicitly adding it to the list item child scope makes it clearer.

PR-URL: https://github.com/nodejs/node/pull/29332


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>