Update to mdbook v0.4.21

Author: badboy

.github/workflows/deploy.yml

@@ -14,7 +14,7 @@ jobs:
     - name: Install mdbook
       run: |
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.13/mdbook-v0.4.13-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.21/mdbook-v0.4.21-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo `pwd`/mdbook >> $GITHUB_PATH
     - name: Generate Book
       run: |