Github Actions: update to mdBook 0.4.14

We need mdBook to be new enough to understand the edition setting ub book.toml.

Author: mattheww

.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/v0.4.5/mdbook-v0.4.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.14/mdbook-v0.4.14-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
         echo "$(pwd)/bin" >> $GITHUB_PATH
     - name: Report versions
       run: |