update the link to mdbook

Technically the old link is still valid because GitHub will redirect, but this is the real location, so we might as well put it directly.

Author: Lokathor

README.md

@@ -23,7 +23,7 @@ infinitesimal fragments of despair.
 
 Building the Nomicon requires [mdBook]. To get it:
 
-[mdBook]: https://github.com/azerupi/mdBook
+[mdBook]: https://github.com/rust-lang-nursery/mdBook
 
 ```bash
 $ cargo install mdbook