fixed ubuntu version (#681)

Author: thuydotm

.github/workflows/pypi-publish.yml

@@ -5,7 +5,7 @@ on:
        - '*'
 jobs:
   publish:
-    runs-on: ubuntu-20-04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@master
       - name: Set up Python 3.9