generate_config: Added hint for FQDN

generate_config.sh

@@ -13,7 +13,7 @@ if [[ -f mailcow.conf ]]; then
 fi
 
 if [ -z "$MAILCOW_HOSTNAME" ]; then
-  read -p "Hostname (FQDN): " -ei "mx.example.org" MAILCOW_HOSTNAME
+  read -p "Hostname (FQDN - example.org is not a valid FQDN): " -ei "mx.example.org" MAILCOW_HOSTNAME
 fi
 
 if [[ -a /etc/timezone ]]; then