Fix reset calculation in forward function

[rG223]

Hi, I found an issue in tutorial 6.

  def forward(self, input_, mem):
    spk = self.surrogate_func((mem-self.threshold))  # call the Heaviside function
    reset = (spk - self.threshold).detach()
    mem = self.beta * mem + input_ - reset
    return spk, mem

The reset computation is incorrect, and it should be reset = (spk * self.threshold).detach().