Use ul tag to avoid duplicated prefixing indexes.

When I tried to use `toc` helper, there are indexes generated by it and also the indexes generated by `ol` tag, resulting something like this:

<ol>
  <li> 1. Level 1 Heading </li>
  <li> 2. Level 1 Heading </li>
  <ol>
    <li> 2.1 Level 2 Heading </li>
  </ol>
</ol>

P.S.
I am trying to add a argument in `toc` helper to limit the levels of headings that it generate. I tried to abort the generation process using `if (level > 2) return;` in several different places inside `toc.js`. But nothing stops it. HELP 😄