po: Update Japanese translation.
* po/ja.po: Fix message with no "%s".
Backport of master commit from: 77e2fcb4ffbad8577a2cf41f17bf92dec6a93ad8
The wrong message caused segmentation fault for key generation when
no expiration is specified.
- GnuPG-bug-id: T3619
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>