Modify the description of N-step buffer

[MrSyee]

N-step buffer code is changed, but the description isn't.

You can see it doesn't actually store a transition in the buffer, unless n_step_buffer is full.

    # in store method
    if len(self.n_step_buffer) < self.n_step:
        return False

It is changed return False -> return ().

[Curt-Park]

@MrSyee Thanks. I pushed the change.
Please take a look.
#23