Update ch04-security.md

[MonsieurPatate]

Edited documentation:

1. Url - https://xtls.github.io/ru/document/level-0/ch04-security.html#4.4 Изменение порта SSH
2. In my case the command systemctl restart ssh did not change the port and in the config file there is said the following:

When systemd socket activation is used (the default), the socket
configuration must be re-generated after changing Port, AddressFamily, or
ListenAddress.

For changes to take effect, run:

systemctl daemon-reload
systemctl restart ssh.socket