Update dotnetcore.yml

- Changed back to just one version of .NET Core.

.github/workflows/dotnetcore.yml

@@ -7,16 +7,11 @@ 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: ${{ matrix.dotnet }}
+          dotnet-version: 3.1
       - name: Build with dotnet
         run: dotnet build --configuration Release