fixed readme

README

 ```
 ip route add ${openvpn_subnet} via 10.116.0.10 src ${eth0_ip}
 sysctl -w net.ipv4.ip_forward=1
-iptables -D FORWARD -j ACCEPT
+iptables -A FORWARD -j ACCEPT
 ```
 
 where `openvpn_subnet` is the subnet configured in `env` (OVPN_SUBNET_ADDR and OVPN_SUBNET_MASK) in CIDR form (x.x.x.x/xx) and `eth0_ip` is the visible ip of the host on the local (physical, not VPN) network.