Fix accidental error in Mastodon nginx config

The well-known folder wasn't served properly anymore.

site-cookbooks/kosmos-mastodon/templates/default/nginx_conf_mastodon.erb

@@ -11,7 +11,7 @@ server {
   access_log "/var/log/nginx/mastodon.access.log";
   error_log "/var/log/nginx/mastodon.error.log";
 
-  location /.well-known/acme-challenge/ { allow all; }
+  location /.well-known { root "/var/www/mastodon"; }
   location / { return 301 https://$host$request_uri; }
 }