Replaced "role=contentinfo" with "role=banner" in "Rules of ARIA attribute usage by HTML element" table (header element) (#502)

Fixes typo / link for allowed implicit role
---------

Co-authored-by: Scott O'Hara <[email protected]>

Author: giacomo-petri

index.html

@@ -1460,7 +1460,7 @@ <h2 id="docconformance">
                 (If not a descendant of an `article`, `aside`, `main`, `nav`
                 or `section` element, or an element with `role=article`, `complementary`,
                 `main`, `navigation` or `region`,
-                then <code>role=<a href="#index-aria-contentinfo">contentinfo</a></code>
+                then <code>role=<a href="#index-aria-banner">banner</a></code>
                 is also allowed, but NOT RECOMMENDED.
                 Otherwise, <code>role=<a href="#index-aria-generic">generic</a></code>
                 is also allowed, but SHOULD NOT be used.)