Link to existing AutoNAT specifications instead of an issue about needing to create said documentation

[crass]

specs/connections/hole-punching.md

Line 161 in 6dd9109

[AutoNAT]: https://github.com/libp2p/specs/issues/180

Replace the link with https://github.com/libp2p/specs/blob/master/autonat/README.md.