Update Japanese translation for "Duration to keep logged in" (#7918)

If this corresponds to "Duration to keep logged in", then current Japanese text says different thing: "Time to store log (not login)".

There are multiple choices for translation of this one and I thought a bit which one is the best, and I'm not sure this is the best, but at least this explains "the time after user logged in until user is logged out", not "the time the log is stored".

app/i18n/ja/admin.php

@@ -167,7 +167,7 @@ return array(
 		),
 		'cookie-duration' => array(
 			'help' => '秒',
-			'number' => 'ログを残す間隔',
+			'number' => 'ログイン状態維持時間',
 		),
 		'force_email_validation' => 'Eメールアドレスの検証を強制します',
 		'instance-name' => 'インスタンス名',