rspec/rules/S4586/how-to-fix-it-dotnet.adoc

Instead of `{keyword_null}` https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.completedtask[`Task.CompletedTask`] or https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.fromresult[`Task.FromResult{typeparameter_TResult}({keyword_null})`] should be returned.