Update mdbook to 0.4.4 in github action

Author: bikeshedder

.github/workflows/mdbook.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v1
         with:
-          mdbook-version: '0.4.1'
+          mdbook-version: '0.4.4'
           # mdbook-version: 'latest'
 
       - run: mdbook build