ci: use latest mdbook

Author: ryzhak

.github/workflows/rbe.yml

@@ -27,7 +27,7 @@ jobs:
     - name: Install mdbook
       run: |
         mkdir bin
-        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.46/mdbook-v0.4.46-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.4.51/mdbook-v0.4.51-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
         echo "$(pwd)/bin" >> ${GITHUB_PATH}
 
     - name: Install mdbook-i18n-helpers