Update CS 228 reference

_includes/footer.html

@@ -1,6 +1,6 @@
 <footer>
 <hr class="slender">
 <div class="credits">
-Site created with <a href="//jekyllrb.com">Jekyll</a> using the <a href="//github.com/clayh53/tufte-jekyll">Tufte theme</a>. Inspired by <a href="//https://ermongroup.github.io/cs228-notes/">Stanford CS 228 Notes</a>.  &copy; {{ site.time | date: '%Y' }}
+Site created with <a href="//jekyllrb.com">Jekyll</a> using the <a href="//github.com/clayh53/tufte-jekyll">Tufte theme</a>. Inspired by <a href="//ermongroup.github.io/cs228-notes/">Stanford CS 228 Notes</a>.  &copy; {{ site.time | date: '%Y' }}
 </div>
 </footer>