correction: allow role=math on img element

closes #523

includes allowance of `role=math` on `img` elements.

Per the ARIA specification:
>While it is not ideal to use an image of a mathematical expression, there exists a significant amount of legacy content where images are used to represent mathematical expressions. Authors SHOULD ensure that images of math are labeled by text that describes the mathematical expression as it might be spoken.

so this role should be allowed for such instances, and any further accessibility gaps caused by the use of the img element would be for authors to resolve beyond the scope of what this spec covers.

Author: scottaohara

index.html

@@ -1585,6 +1585,9 @@ <h2 id="docconformance">
                 <a href="#index-aria-button">`button`</a>,
                 <a href="#index-aria-checkbox">`checkbox`</a>,
                 <a href="#index-aria-link">`link`</a>,
+                <span class="correction">
+                  <a href="#index-aria-math">`math`</a>,
+                </span>
                 <a href="#index-aria-menuitem">`menuitem`</a>,
                 <a href="#index-aria-menuitemcheckbox">`menuitemcheckbox`</a>,
                 <a href="#index-aria-menuitemradio">`menuitemradio`</a>,
@@ -1601,7 +1604,8 @@ <h2 id="docconformance">
                 <a href="#index-aria-slider">`slider`</a>,
                 <a href="#index-aria-switch">`switch`</a>,
                 <a href="#index-aria-tab">`tab`</a> or
-                <a href="#index-aria-treeitem">`treeitem`</a>. (<code><a href="#index-aria-img">img</a></code> is also allowed, but NOT RECOMMENDED.)
+                <a href="#index-aria-treeitem">`treeitem`</a>. 
+                (<code><a href="#index-aria-img">img</a></code> is also allowed, but NOT RECOMMENDED.)
               </p>
               <p>
                 DPub Role: