Updated Prerequisites

The file was edited to remove the outdated (and in some cases removed) `netstat` command and replace it with `ss`

docs/prerequisite-system.md

@@ -31,7 +31,7 @@ ClamAV and Solr are greedy RAM munchers. You can disable them in `mailcow.conf`
 Please check if any of mailcow's standard ports are open and not in use by other applications:
 
 ```
-# netstat -tulpn | grep -E -w '25|80|110|143|443|465|587|993|995|4190'
+# ss -tlpn | grep -E -w '25|80|110|143|443|465|587|993|995|4190'
 ```
 
 !!! warning