Correction: adds missing radio role allowance for img

closes #380.
As noted in #212, `role=radio` was meant to be allowed on an `img alt=foo`.  This corrects the mistake of that allowance not making it into the spec.

index.html

@@ -1185,6 +1185,9 @@ <h2 id="docconformance">
                 <a href="#index-aria-menuitemradio">`menuitemradio`</a>,
                 <a href="#index-aria-option">`option`</a>,
                 <a href="#index-aria-progressbar">`progressbar`</a>,
+                <span class="proposed correction">
+                  <a href="#index-aria-radio">`radio`</a>,
+                </span>
                 <a href="#index-aria-scrollbar">`scrollbar`</a>,
                 <a href="#index-aria-separator">`separator`</a>,
                 <a href="#index-aria-slider">`slider`</a>,