doc: use semantically appropriate tag for lines

The HTML <hr> element represents a thematic break between
paragraph-level elements: for example, a change of scene in a story,
or a shift of topic within a section.
The HTML Content Division element (<div>) is the generic container
for flow content and does not inherently represent anything.

The <hr> tag semantically fits better the meaning of the line between
the lists.

Refs: https://html.spec.whatwg.org/multipage/grouping-content.html#the-hr-element

PR-URL: https://github.com/nodejs/node/pull/34660


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>