Add a hint about opening the book offline

Author: mo8it

website/content/setup/index.md

@@ -14,7 +14,7 @@ This will also install _Cargo_, Rust's package/project manager.
 >
 > Debian: `sudo apt install gcc`\
 > Fedora: `sudo dnf install gcc`
-
+>
 > 🍎 If you are on **MacOS**, make sure you have _Xcode and its developer tools_ installed: `xcode-select --install`
 
 ## Installing Rustlings
@@ -75,7 +75,7 @@ On Windows, we recommend the [Windows Terminal](https://aka.ms/terminal).
 
 ### Offline documentation
 
-Whenever you're working on Rustlings offline, you can access a local copy of the standard library documentation by running `rustup doc --std`.
+Whenever you're working on Rustlings offline, you can access a local copy of the book or the standard library documentation by running `rustup doc --book` or `rustup doc --std`.
 
 ## Usage