That's a Windows path separator

Author: CodeCasterNL

.github/workflows/Publish-Package.yml

@@ -42,4 +42,4 @@ jobs:
 
     - name: Release to NuGet.org
       if: startsWith(github.ref, 'refs/tags/')
-      run: nuget push **\*.nupkg -Source 'https://api.nuget.org/v3/index.json' -ApiKey ${{secrets.NUGET_API_KEY}}
+      run: nuget push **/*.nupkg -Source 'https://api.nuget.org/v3/index.json' -ApiKey ${{secrets.NUGET_API_KEY}}