Pull in unreleased mdbook fixes

Author: dtolnay

.github/workflows/site.yml

@@ -16,9 +16,9 @@ jobs:
 
       - name: Get mdBook
         run: |
-          export MDBOOK_VERSION="v0.4.4"
+          export MDBOOK_VERSION="dtolnay"
           export MDBOOK_TARBALL="mdbook-${MDBOOK_VERSION}-x86_64-unknown-linux-gnu.tar.gz"
-          export MDBOOK_URL="https://github.com/rust-lang/mdBook/releases/download/${MDBOOK_VERSION}/${MDBOOK_TARBALL}"
+          export MDBOOK_URL="https://github.com/rust-lang/mdBook/releases/download/cxx/${MDBOOK_TARBALL}"
           curl -Lf "${MDBOOK_URL}" | tar -xzC book
 
       - name: Build