Fix typo, modify Japanese expression

Fix typo, modify Japanese expression.
"復元" also means decrypt in Japanese, however more precisely, the word
"復号" is used as the technical term.
(People often use "復号化" but it is not correct when I split hairs;))

ja/ebook/preface.md

@@ -60,7 +60,7 @@
  - 9.3 [XSS攻撃の回避](09.3.md)
  - 9.4 [SQLインジェクションの回避](09.4.md)
  - 9.5 [パスワードの保存](09.5.md)
- - 9.6 [データを暗号化/復元する](09.6.md)
+ - 9.6 [データを暗号化/復号する](09.6.md)
  - 9.7 [まとめ](09.7.md)
 * 10.[国際化とローカライズ](10.0.md) 
  - 10.1 [デフォルトロケールの設定](10.1.md)