ja_JP: Change the offset for Taisho gan-nen from 2 to 1 [BZ #24162]
The offset in era-string format for Taisho gan-nen (1912) is currently
defined as 2, but it should be 1. So fix it. "Gan-nen" means the 1st
(origin) year, Taisho started on July 30, 1912.