add ref to github checkout step

Author: Rio6

.github/workflows/jupyter-notebook.yml

@@ -12,6 +12,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: ${{github.ref}}
+
       - uses: actions/setup-python@v1
         with:
           python-version: 3.7