Update class-wc-countries.php for Japanese State

I don't use the word of Province for the largest administrative divisions of Japan, like the state in USA.
Sometimes I use the word of State for 都道府県. But I search some site, It seems to say that "Prefecture"
 is formally. Perhaps Prefecture is best.

Please check following site.
1. One of Most Popular Japanese to English dictionary site:
http://ejje.weblio.jp/content/%E9%83%BD%E9%81%93%E5%BA%9C%E7%9C%8C

2. Official Japanese law Translation site(Please click 都道府県)
http://www.japaneselawtranslation.go.jp/dict/list?re=02&ft=1&dn=1&ky=%E9%83%BD%E9%81%93%E5%BA%9C%E7%9C%8C&x=63&y=8&co=1
Merge pull request #5730 from shoheitanaka/patch-4

Update class-wc-countries.php for Japanese State