use /demo like it says in the instructions

https://github.com/Johni0702/mumble-web/issues/24

README.md

@@ -72,7 +72,7 @@ server {
         location / {
                 root /path/to/dist;
         }
-        location /mumble {
+        location /demo {
                 proxy_pass http://websockify:64737;
                 proxy_http_version 1.1;
                 proxy_set_header Upgrade $http_upgrade;