_s: Use <main> element for main content.

See: http://www.w3.org/TR/html51/grouping-content.html#the-main-element

Props @BFTrick for initial patch.
Fixes #222.

404.php

@@ -7,8 +7,9 @@
 
 get_header(); ?>
 
+
 	<div id="primary" class="content-area">
-		<div id="content" class="site-content" role="main">
+		<main id="main" class="site-main" role="main">
 
 			<section class="error-404 not-found">
 				<header class="page-header">
@@ -50,7 +51,7 @@ get_header(); ?>
 				</div><!-- .page-content -->
 			</section><!-- .error-404 -->
 
-		</div><!-- #content -->
+		</main><!-- #main -->
 	</div><!-- #primary -->
 
 <?php get_footer(); ?>