example: fix key type of xdp example

[BillyChen1]

The IP address output by xdp demo is empty. Although it can be solved by directly changing the key type to netip.Addr after go1.18 version, I still use a more general way to fix it.
Issue: #1175

[rgo3]

Thanks for opening a PR! Given that the library requires Go 1.20 to be the minimum version I don't see a real benefit in going with the more general way. Just using netip.Addr looks cleaner and would also work if someone wants to extend the example with ipv6 support.

[BillyChen1]

@rgo3 done

[rgo3]

LGTM!

I've noticed that this broke once we merged 4609dc7 and it's likely that fixing the internals would probably prevent us from making this change necessary. But switching to netip.Addr seems sensible either way as the only reason we didn't use it in the first place was that 1.18 wasn't the minimum required Go version.

[lmb]

Interesting, netip.Addr works as a key since it implements BinaryUnmarshaler!