refactor: Format book.toml with consistent indentation

book.toml

@@ -1,18 +1,13 @@
 [book]
 title = "Rust 程序设计语言 简体中文版"
-authors = [
-    "Steve Klabnik",
-    "Carol Nichols",
-    "Chris Krycho",
-    "来自 Rust 社区的贡献（Rust 中文社区翻译）",
-]
+authors = ["Steve Klabnik", "Carol Nichols", "Chris Krycho", "来自 Rust 社区的贡献（Rust 中文社区翻译）"]
 
 [output.html]
 additional-css = [
-    "ferris.css",
-    "theme/2018-edition.css",
-    "theme/semantic-notes.css",
-    "theme/listing.css",
+  "ferris.css",
+  "theme/2018-edition.css",
+  "theme/semantic-notes.css",
+  "theme/listing.css",
 ]
 additional-js = ["ferris.js"]
 git-repository-url = "https://github.com/KaiserY/trpl-zh-cn/tree/main"
@@ -25,6 +20,7 @@ custom-template = "custom-template.typ"
 pdf = true
 section-number = true
 rust-book = true
+optional = true
 
 [rust]
 edition = "2024"