teacher student sigmoid loss

test=develop

python/paddle/fluid/layers/nn.py

@@ -9204,7 +9204,7 @@ def teacher_student_sigmoid_loss(input,
                                 by the previous operator.
         label (Variable|list):  the ground truth which is a 2-D tensor with
                                 shape [N x 1], where N is the batch size.
-        soft_max_up_bound (float):  if input > soft_max_up_bound, will be bound 
+        soft_max_up_bound  (float):  if input > soft_max_up_bound, will be bound 
         soft_max_lower_bound (float): if input < soft_max_lower_bound, will be bound
 
     Returns: