Update mdbook

- Changelog to 0.4.5: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-045)
- mdbook security advisory: https://blog.rust-lang.org/2021/01/04/mdbook-security-advisory.html

Some breaking changes were introduced in 0.4.0 but I skimmed The References built by 0.4.5 and no regression detected.

Author: weihanglo

.github/workflows/main.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Install mdbook
       run: |
         mkdir bin
-        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.5/mdbook-v0.3.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.5/mdbook-v0.4.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
         echo "$(pwd)/bin" >> $GITHUB_PATH
     - name: Report versions
       run: |