H5: Fix typo in a command

exercises/h5.md

@@ -187,7 +187,7 @@ Executing /lib/systemd/systemd-sysv-install enable ufw
 ```
 sudo systemctl stop ufw.service && sudo systemctl disable ufw.service
 sudo apt-get purge --remove ufw
-sudo iptables --flush && sudo iptables --delete-chains
+sudo iptables --flush && sudo iptables --delete-chain
 ```
 
 **2.** Confirm deletion of ufw entries from iptables firewall rule list with