Update .drone.yml

.drone.yml

@@ -11,6 +11,7 @@ steps:
   commands:
 #  - pip install -U -r ./requirements.txt
   - pip install --force-reinstall mkdocs
+  - pip install pymdown-extensions
   - mkdocs build --clean -f ./mkdocs.yml
   - cp -r site/* /site/
   - chmod -R 777 /site