allow to quickly edit a page directly on github

Author: tshepang

book.toml

@@ -6,6 +6,7 @@ author = "The Rust Project Developers"
 [output.html]
 additional-css = ["theme/reference.css"]
 git-repository-url = "https://github.com/rust-lang/reference/"
+edit-url-template = "https://github.com/rust-lang/reference/edit/master/{path}"
 
 [output.html.redirect]
 "/expressions/enum-variant-expr.html" = "struct-expr.html"