algorithm error generating aria-level for caution headings

Seems, there is a coding error in the realization of heading levels for caution-notes on some W3C sites:

epub-specs/epub33/common/js/add-caution-hd.js

Line 16 in 181828b

caution_hd.setAttribute('aria-level', parent_num+1);

Example:
https://www.w3.org/TR/epub-33/#sec-intro-relations
aria-heading with an extraordinary value 31 for aria-level:
<div class="caution" id="caution-0">
<div
role="heading"
aria-level="31"
class="caution-title marker">
<span>Caution</span>
</div>

Two digits concatenated together, but should be added mathematically.

This seems to be a class-1 modification request. I also believe it is a valid Erratum, so we should mark it as such

@wareid @shiestyle @TzviyaSiegman @dauwhe