Fetch mdBook in new location

The mdBook project was moved to the rust-lang organization, out of the
nursery organization.

rust-lang/mdBook#1080
rust-lang/mdBook#1083
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-034

Author: t-nelis

azure-pipelines.yml

@@ -98,7 +98,7 @@ jobs:
     - bash: |
         set -e
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang-nursery/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo "##vso[task.prependpath]`pwd`/mdbook"
       displayName: "Install mdbook"
     - bash: cargo doc --no-deps