Fix a markdown link in README

Author: giraffate

doc/README.md

@@ -4,7 +4,7 @@ This directory contains rustup's documentation.
 
 ## Building the book
 
-Building the book requires [mdBook]. To get it:
+Building the book requires [mdBook](https://github.com/rust-lang/mdBook). To get it:
 
 ```console
 $ cargo install mdbook