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/09.5.md

@@ -86,4 +86,4 @@ Go言語のこの三種類の暗号化アルゴリズムの実装は以下の通
 ## links
    * [目次](<preface.md>)
    * 前へ: [入力のフィルタリングを確実に行う](<09.4.md>)
-   * 次へ: [データを暗号化/復元する](<09.6.md>)
+   * 次へ: [データを暗号化/復号する](<09.6.md>)