Fix slack link

Author: john-b-yang

templates/_sidebar.html

@@ -48,7 +48,7 @@
             <i class="fab fa-discord"></i>
         </a>
         &nbsp;
-        <a href="" target="https://join.slack.com/t/swe-bench/shared_invite/zt-36pj9bu5s-o3_yXPZbaH2wVnxnss1EkQ" rel="noopener noreferrer">
+        <a href="https://join.slack.com/t/swe-bench/shared_invite/zt-36pj9bu5s-o3_yXPZbaH2wVnxnss1EkQ" target="_blank" rel="noopener noreferrer">
             <i class="fab fa-slack"></i>
         </a>
         <button id="theme-toggle" class="theme-toggle" aria-label="Toggle dark mode">