ssl on is obsolete (#194)

http://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl

docs/firststeps-rp.md

@@ -98,11 +98,10 @@ server {
   return 301 https://$host$request_uri;
 }
 server {
-  listen 443;
+  listen 443 ssl;
-  listen [::]:443;
+  listen [::]:443 ssl;
   server_name CHANGE_TO_MAILCOW_HOSTNAME autodiscover.* autoconfig.*;
 
-  ssl on;
   ssl_certificate MAILCOW_PATH/data/assets/ssl/cert.pem;
   ssl_certificate_key MAILCOW_PATH/data/assets/ssl/key.pem;
   ssl_protocols TLSv1 TLSv1.1 TLSv1.2;