message: update ref link for docs
diff: @@ -1742,7 +1742,7 @@ def adjust_gamma(image, gamma=1, gain=1):
  For gamma less than 1, the histogram will shift towards right and
  the output image will be brighter than the input image.
  References:
  - [1] http://en.wikipedia.org/wiki/Gamma_correction
  + [Wikipedia](http://en.wikipedia.org/wiki/Gamma_correction)
  """
  with ops.name_scope(None, 'adjust_gamma', [image, gamma, gain]) as name:
  