fix delay update bug (#281

fix delay update bug

Author: chaoming0625

brainpy/math/delayvars.py

@@ -445,8 +445,8 @@ def update(self, value: Union[float, int, bool, JaxArray, jnp.DeviceArray]):
       The value of the latest data, used to update this delay variable.
     """
     if self.update_method == ROTATION_UPDATING:
+      self.idx.value = stop_gradient((self.idx - 1) % self.num_delay_step)
       self.data[self.idx[0]] = value
-      self.idx.value = stop_gradient((self.idx + 1) % self.num_delay_step)
 
     elif self.update_method == CONCAT_UPDATING:
       if self.num_delay_step >= 2: