book.toml: add Git repository URL

The repository is now public [1], thus add the Git repository URL.

Link: https://lore.kernel.org/rust-for-linux/CANiq72kMS8UoYpg0hW1=UDAAkytEoeyqGzg+Sys8VSaucr+HzQ@mail.gmail.com/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

book.toml

@@ -8,7 +8,7 @@ language = "en"
 create-missing = false
 
 [output.html]
-git-repository-url = ""
+git-repository-url = "https://github.com/Rust-for-Linux/rust-for-linux.com"
 edit-url-template = ""
 no-section-label = true