Update docs.yml

.github/workflows/docs.yml

@@ -22,7 +22,7 @@ jobs:
           cache: 'pip'
       - name: Install dependencies
         run: |
-          python3 -m pip install --upgrade pip pygments bs4
+          python3 -m pip install --upgrade pip pygments bs4 lxml
           python3 -m pip install git+https://github.com/capjamesg/aurora
           pip install -e .
           cd docs