[bugfix] Zulip link restored.

Keyboard slip caused it to revert to the old Gitter url.

index.html

@@ -71,7 +71,7 @@
       <span class="jscoq-name">jsCoq</span> is open source. If you find any problem or want to make
       any contribution, you are extremely welcome! We await your
       feedback at <a href="https://github.com/jscoq/jscoq">GitHub</a>
-      and <a href="https://gitter.im/jscoq/Lobby">Gitter</a>.
+      and <a href="https://coq.zulipchat.com/#narrow/stream/256336-jsCoq">Zulip</a>.
     </p>
     <h4>Instructions:</h4>
     <p>