`transition_to` spec limits state to String type

[kyleboe]

In the spec definition for transition_to it specifies String.t() as the next_state's type.

machinery/lib/machinery.ex

Line 94 in 4e7ae81

@spec transition_to(struct, module, String.t()) :: {:ok, struct} | {:error, String.t()}

I believe this is incorrect. An atom is used in examples & typespecs for the state argument(s). I also confirmed locally that using atom states does not cause any errors.

Should a typespec be added for state? Possibly something like so:

@typedoc """
A state should be a string or an atom. Whichever corresponds with the underlying state field on the given struct.
"""
@type state :: atom | String.t()