CI: Use sphinx-toolbox fork of sphinx-action

Author: ZedThree

.github/workflows/docs.yml

@@ -5,8 +5,8 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
-      - uses: ammaraskar/sphinx-action@master
+      - uses: actions/checkout@v2
+      - uses: sphinx-toolbox/sphinx-action@master
         with:
           pre-build-command: "pip install .[docs]"
           docs-folder: "docs/"