Update mdbook links (#1989)

Update mdbook links

Author: carols10cents

CONTRIBUTING.md

@@ -39,4 +39,4 @@ a new language! We're waiting on [mdbook support] for multiple languages
 before we merge any in, but feel free to start!
 
 [Translations]: https://github.com/rust-lang/book/issues?q=is%3Aopen+is%3Aissue+label%3ATranslations
-[mdbook support]: https://github.com/azerupi/mdBook/issues/5
+[mdbook support]: https://github.com/rust-lang-nursery/mdBook/issues/5

README.md

@@ -22,7 +22,7 @@ releases are updated less frequently.
 Building the book requires [mdBook], ideally the same 0.2.x version that
 rust-lang/rust uses in [this file][rust-mdbook]. To get it:
 
-[mdBook]: https://github.com/azerupi/mdBook
+[mdBook]: https://github.com/rust-lang-nursery/mdBook
 [rust-mdbook]: https://github.com/rust-lang/rust/blob/master/src/tools/rustbook/Cargo.toml
 
 ```bash
@@ -77,7 +77,7 @@ a new language! We're waiting on [mdbook support] for multiple languages
 before we merge any in, but feel free to start!
 
 [Translations]: https://github.com/rust-lang/book/issues?q=is%3Aopen+is%3Aissue+label%3ATranslations
-[mdbook support]: https://github.com/azerupi/mdBook/issues/5
+[mdbook support]: https://github.com/rust-lang-nursery/mdBook/issues/5
 
 ## Graphviz dot