Fix incorrect variable name in Sample/Tutorial4

[sssurvey]

While going over the sample tutorials. I noticed there is a minor issue in Tutorial4.md.

Quote from the current tutorial:

Because TodoWorkflow.State has no properties anymore, it can't be a data class, so we need to change it to an object. Since the only reason to have a custom type for state is to define the data we want to store, we don't need a custom type anymore so we can just use Unit. You might ask why we need a state at all now. We will discuss that in the next section. For now Unit will get us moving forward.

However, based on the context, it seems that it should be TodoListWorkflow.State instead of TodoWorkflow.State. (In this tutorial, we are refactoring TodoListWorkflow by introducing a new TodoWorkflow)

[CLAassistant]

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