message: Fix activation argument document for ConvLSTM2D
  In the example the activation = 'tanh'
  whereas the documentation says no activation is applied (ie. "linear" activation: a(x) = x).
diff: @@ -786,8 +786,8 @@ class ConvLSTM2D(ConvRNN2D):
  Currently, specifying any `dilation_rate` value != 1 is
  incompatible with specifying any `strides` value != 1.
  activation: Activation function to use.
  - If you don't specify anything, no activation is applied
  - (ie. "linear" activation: `a(x) = x`).
  + By default hyperbolic tangent activation function is applied
  + (`tanh(x)`).
  recurrent_activation: Activation function to use
  for the recurrent step.
  use_bias: Boolean, whether the layer uses a bias vector.
  