doc: fix some recent nits in assert.md

* add missing bottom reference
* fix possible typos
* fix ABC-order in references and sections

PR-URL: https://github.com/nodejs/node/pull/19284


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>