message: Fix invalid string formatting in merge.py
diff: @@ -500,7 +500,7 @@ class Concatenate(_Merge):
  if len(shape_set) != 1:
  err_msg = ('A `Concatenate` layer requires inputs with matching shapes '
  'except for the concat axis. Got inputs shapes: %s' %
  - input_shape)
  + (input_shape,))
  # Make sure all the shapes have same ranks.
  ranks = set(len(shape) for shape in shape_set)
  if len(ranks) != 1:
  