fix nginx example config to use new name

etc/nginx/conf.d/bookmark-archiver.conf → etc/nginx/conf.d/archivebox.conf

@@ -2,7 +2,7 @@ server {
     listen                      80 default_server;
     server_name                 _;
 
-    root                        /data/output/;
+    root                        /opt/ArchiveBox/output/;
     index                       index.html;
     autoindex                   on;