Typo on Workflows File

[BlueWings98]

There is a Typo on the workflows file that may suggest that those tests are not being run.
- name: Test on .NET Core 3.1 run: dotnet test src/RestClient.Net.UnitTests --no-build --verbosity normal --framework nercoreapp3.1
Instead of "nercoreapp3.1" it should say "netcoreapp3.1"
This is of great importance as it ensures compatibility with that legacy code.