Update dotnetcore.yml

Learning about continuous integration.
- Testing build using multiple versions of .NET Core.

.github/workflows/dotnetcore.yml

@@ -7,11 +7,16 @@ jobs:
 
     runs-on: ubuntu-latest
 
+    strategy:
+      matrix:
+        dotnet: [ '2.1', '3.1' ]
+    name: Dotnet ${{ matrix.dotnet }} build
+
     steps:
       - uses: actions/checkout@v1
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with:
-        dotnet-version: 2.2.108
+          dotnet-version: ${{ matrix.dotnet }}
       - name: Build with dotnet
         run: dotnet build --configuration Release