Changed dotnet build path

Author: codingben

.github/workflows/build.yml

@@ -13,6 +13,6 @@ jobs:
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: '2.1'
+          dotnet-version: '2.1.x'
       - name: Box2D Build
-        run: dotnet build src/ -c Release
+        run: dotnet build src/box2dx/Box2D.NetStandard -c Release