Update dotnetcore.yml

Author: darkl

.github/workflows/dotnetcore.yml

@@ -13,8 +13,8 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: |
-            '9.0.x'
-            '3.1.x'
+            9.0.x
+            3.1.x
     - name: Build with dotnet
       working-directory: src/netstandard/
       run: dotnet build --configuration Release