Use <small> for the copyright

This follows the new semantic meaning of <small> in HTML 5

Quoting the spec:
> Small print typically features disclaimers, caveats, legal restrictions,
> or copyrights. Small print is also sometimes used for attribution, or
> for satisfying licensing requirements.

_layouts/default.html

@@ -16,9 +16,9 @@
       {{ content }}
 
       <div class="footer">
-        <p>
+        <small>
           &copy; <time datetime="{{ site.time | date_to_xmlschema}}">{{ site.time | date: '%Y' }}</time>. All rights reserved.
-        </p>
+        </small>
       </div>
     </div>