Update mdbook to 0.4.26 (#500)

This fixes a compilation problem when deploying Timely.

Signed-off-by: Moritz Hoffmann <antiguru@gmail.com>

Author: antiguru

.github/workflows/deploy.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
-      - run: cargo install mdbook --version 0.4.20
+      - run: cargo install mdbook --version 0.4.26
       - run: cd mdbook && mdbook build
       - uses: JamesIves/github-pages-deploy-action@v4
         with: