Fix deploy path

When adding the linkchecker renderer, that changed mdbook to give each
renderer a separate path.

Author: ehuss

.github/workflows/deploy.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v3
         with:
-          path: ./book
+          path: ./book/html
 
   deploy:
     if: github.repository_owner == 'rust-lang'