docs(book): fix site-url path format (#4)

Author: Atliac

book.toml

@@ -7,7 +7,7 @@ language = "en"
 [output.html]
 default-theme = "rust"
 git-repository-url = "https://github.com/atliac/rust-macros-tutorial"
-site-url = "/rust-macros-tutorial"
+site-url = "/rust-macros-tutorial/"
 
 [output.html.playground]
 editable = true