fix: update cispa phd application link

Author: fabian-thomas

layouts/partials/footer.html

@@ -24,7 +24,7 @@
       </div>
     </div>
     <p style="text-align: center; color: gray;">
-      Want to contribute to such research? <a href="https://career.cispa.de/phd.html">CISPA is constantly looking for PhD students</a><br><br>
+      Want to contribute to such research? <a href="https://cispa.de/en/career/phd-at-cispa">CISPA is constantly looking for PhD students</a><br><br>
       Last Update: {{ time.Now | time.Format "2006-01-02" }}, <a href="https://github.com/ghostwriteattack/ghostwriteattack.github.io">Changelog and website repository</a>, Designed by <a href="https://leon.trampert.me">Leon Trampert</a><br><br>
       {{ .Site.Copyright }}
     </p>