HTML5 validation outline error: heading level skipped on the captcha preference page

First found in CI:

```
Validation errors in /xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=captcha
ERROR: The heading “h3” (with computed level 3) follows the heading “h1” (with computed level 1), skipping 1 heading level. at line [1101] column [128]

Extracts of the invalid content:

1099 <input value="Save" class='button' type='submit'>
1100 </div>
1101 </form><div class="row"><div class="col-xs-12 col-lg-7"><h3 id="HConfigureJCaptcha" class="wikigeneratedid wikigeneratedheader"><span>Configure JCaptcha</span></h3><form action="/xwiki/bin/save/XWiki/Captcha/JCaptcha/Configuration" class='xform jcaptcha'>
1102 <dl>
1103 <dt>
```