Fix contents insertion

Author: ericthelemur

book.toml

@@ -24,5 +24,5 @@ line-numbers = true
 [preprocessor.toc]
 command = "mdbook-toc"
 renderer = ["html"]
-marker = "[[_TOC_]]"
+marker = "[[TOC]]"
 max-level = 2

src/raytracer.md

@@ -12,7 +12,7 @@ Also, unlike the original book, I've not included the code snippets inline, as t
 
 ## Contents
 
-[[_TOC_]]
+[[TOC]]
 
 ## 1: Images