Update dotnetcore.yml

.github/workflows/dotnetcore.yml

@@ -4,9 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
-
     steps:
     - uses: actions/checkout@v1
       with:
@@ -14,6 +12,6 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.100
+        dotnet-version: 5.0.x
     - name: Build and test with dotnet
       run: dotnet test --configuration Release