message: Update activations.py
  Changed external reference link for hard_sigmoid to wikipedia
diff: @@ -435,7 +435,7 @@ def hard_sigmoid(x):
  A faster approximation of the sigmoid activation.
  Piecewise linear approximation of the sigmoid function.
  - Ref: 'https://paperswithcode.com/method/hard-sigmoid'
  + Ref: 'https://en.wikipedia.org/wiki/Hard_sigmoid'
  For example:
  