part2 exercise 2.18 countries API

[amylovesong]

The REST API https://studies.cs.helsinki.fi/restcountries/ in the exercise now is only supporting all countries query or a full name query, it isn't match the exercise's requirement.

I'v find another API https://restcountries.com/#rest-countries from this project's issues, and it's ok to use!

Just share it here if someone else need it, have fun! 🙂

BTW, the course is awesome! 🚀

Thanks a lot! ☕️