Update to H91 per LC2877

I've made edits to address the comment in LC-2877, which was essentially that the H91 technique (http://www.w3.org/TR/WCAG20-TECHS/H91.html) needs to include type=file.  I've verified the information necessary to complete the table and made the changes in a pull request from my branch to the current working branch.  You can review the changes here:

https://github.com/w3c/wcag/pull/58/files?diff=split

Thanks,
AWK

Andrew Kirkpatrick
Group Product Manager, Accessibility
Adobe Systems

akirkpat@adobe.com<mailto:akirkpatrick@adobe.com>
http://twitter.com/awkawk
http://blogs.adobe.com/accessibility