Fix internet browsing vpn example forward rule

[lvaroqui]

Hello,

I believe an iptables FORWARD rule is missing in example-internet-browsing-vpn.

If the global FORWARD policy is DROP, I believe that we need to accept forwarding connections from VPN (wg0) to Ethernet interface (eth0).

[pirate]

this is optional and depends on what you're doing with the VPN (e.g. hosting a service), but I think it's ok to merge