Fix broken demo on Kalman Smoothing with neural network driven dynamics

[albertpod]

Basic Kalman Smoothing with LSTM driven dynamics doesn't work.

It breaks with the following error:

MethodError: no method matching Flux.LSTMCell(::Matrix{Float64}, ::Matrix{Float64}, ::Vector{Float64}, ::Vector{Float64}, ::Vector{Float64})
Closest candidates are:
  Flux.LSTMCell(::A, ::A, ::V, ::S) where {A, V, S} at ~/.julia/packages/Flux/BPPNj/src/layers/recurrent.jl:142

Must be updated.

P.S. For this example, we can use MP instead of LSTM.