Fix os name typo.

.github/workflows/pythonpublish.yml

@@ -16,8 +16,8 @@ jobs:
         os: [macos-latest, windows-latest]
         python-version: ['3.5', '3.6', '3.7', '3.8']
         include:
-         - os: linux-latest
-           python-version: '3.x'
+          - os: ubuntu-latest
+            python-version: '3.x'
 
     steps:
     - uses: actions/checkout@v2