Add Ryujin-onsen Motoyu to the Wakayama12 achievement

This onsen is not formally included on the Wakayama12 site in the achievement, but there is an achievement poster hanging there and in practice they sell the onsen stamp itself for Wakayama12. It may be worth adding this to the achievement with some additional note.
https://onsenoni.com/ru/places/6876fd9a18-ryujin-onsen-motoyu