set ARIA properties in navbar

_layouts/default.html

@@ -18,7 +18,7 @@
 		<nav>
 			<ul>
 				{% for item in site.data.navigation %}
-					<li{% if page.url == item.link %} class="current"{% endif %}>
+					<li {% if page.url == item.link %}class="current" aria-current="true"{% else %}aria-current="false"{% endif %}>
 						<a href="{{ item.link }}">{{ item.name }}</a>
 					</li>
 				{% endfor %}