docs: fix ws.publish (#6558)

In this example there is no server variable in the context, and here it makes more sense to use ws.publish. It is explained below that once the serve is done, the server.publish can be used.

docs/api/websockets.md

@@ -192,7 +192,7 @@ const server = Bun.serve<{ username: string }>({
     close(ws) {
       const msg = `${ws.data.username} has left the chat`;
       ws.unsubscribe("the-group-chat");
-      server.publish("the-group-chat", msg);
+      ws.publish("the-group-chat", msg);
     },
   },
 });