[21 Techniques index page] fix text : Failures -> Techniques

[shawna-slh]

For https://www.w3.org/WAI/WCAG21/Techniques/ , in the page contents list, change
<li><a href="#general">General Failures</a></li>
to
<li><a href="#general">General Techniques</a></li>

(as in #3109 which fixed https://www.w3.org/WAI/WCAG22/Techniques/)

[bruce-usab]

Ping to @iadawn via @avkuo and the wcag2-issues tf. The correction is merged, but change is not public facing. Please advise.

[kfranqueiro]

This is resolved now that WCAG 2.1 Techniques and Understanding Docs have been republished via the new build system, made possible by #4007.