Update gh-pages.yml

Author: simongravelle

.github/workflows/gh-pages.yml

@@ -23,6 +23,9 @@ jobs:
           submodules: recursive
           fetch-depth: 0
 
+      - name: Checkout submodules
+        run: git submodule update --init --recursive
+
       - name: Set up Python
         uses: actions/setup-python@v2.2.1
         with: