ci.yml going back to ubuntu 20.04 for Docs

Author: cfuentes-dbx

.github/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
         run: |
           pytest -v test/unit/test_dropbox_unit.py
   Docs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python environment