text: Title: Scattering2D docstring contains an outdated example
  Body: See
  https://github.com/kymatio/kymatio/blob/master/kymatio/scattering2d/scattering2d.py#L37
  
  
  ```python
          # 1) Define a Scattering object as:
          s = Scattering2D(J, M, N)
          #    where (M, N) are the image sizes and 2**J the scale of the scattering
  ```
  
  
  The constructor should read as `Scattering2D(J, shape=(M, N))`
  
  