Doc: Fix broken ProgramBench banner link (#837)

Author: Chesars

docs/overrides/main.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 
 {% block announce %}
-<a href="{{ config.site_url }}/latest/usage/programbench/">
+<a href="{{ 'usage/programbench/' | url }}">
   🎉 Run mini-swe-agent on our new & extremely challenging benchmark, ProgramBench
 </a>
 {% endblock %}