Update main.yml

Fix failing build

Author: paulswithers

.github/workflows/main.yml

@@ -39,7 +39,8 @@ jobs:
         run: |
           pip install mkdocs-material
           pip install mkdocs-awesome-pages-plugin
+          pip install mkdocs-section-index
       # Do the actual deployment
       - name: Deploy GH Pages
         run: |
-          mkdocs gh-deploy --force
+          mkdocs gh-deploy --force