update link to tshirt

Author: ys

_includes/footer.html

@@ -1,7 +1,7 @@
 <div class="footer">
   <div class="contact">
     <p>
-      <a href="/tees">Want a tshirt?</a><br>
+      <a href="https://rubyburgers.spreadshirt.be/">Want a tshirt or a mug?</a><br>
       <a href="http://github.com/rubyburgers/">github.com/rubyburgers</a><br />
       <a href="http://twitter.com/ruby_burgers/">twitter.com/ruby_burgers</a><br />
     </p>