Fix error in Github action

Error seen in Github action at https://github.com/elm-fullstack/elm-fullstack/actions/runs/149720732:

> Invalid workflow file
> You have an error in your yaml syntax on line 23

.github/workflows/test-and-publish.yml

@@ -21,7 +21,7 @@ jobs:
     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: '3.1.301'
-  - name: Try use dotnet core 3
+    - name: Try use dotnet core 3
       run: dotnet --info
     - uses: actions/checkout@v1
     - name: Run tests