message: Update mdbook to v0.4
  v0.3.7 no longer compiles as of Rust 1.64,
  see
diff: @@ -31,7 +31,7 @@ jobs:
  - name: Install mdbook
  run: |
  (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.3" mdbook)
  + (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.4" mdbook)
  cargo install-update -a
  - name: Build book
  