chore: bump mdbook to 0.4.27

Previously mdbook was bumped in rust-lang#11646 for contrib.yml worflow
but forgot main.yaml workflow. This makes the two in sync and also
upgrade to the latest 0.4.27.
(Though there is nothing really changed for application users as us)

.github/workflows/contrib.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Install mdbook
       run: |
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.26/mdbook-v0.4.26-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.27/mdbook-v0.4.27-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo `pwd`/mdbook >> $GITHUB_PATH
     - name: Deploy docs
       run: |

.github/workflows/main.yml

@@ -170,7 +170,7 @@ jobs:
     - run: cd src/doc/semver-check && cargo +stable run
     - run: |
         mkdir mdbook
-        curl -Lf 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=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.27/mdbook-v0.4.27-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo `pwd`/mdbook >> $GITHUB_PATH
     - run: cargo doc --document-private-items --no-deps
       env: