Update the class imbalance POC

Currently the example I give  in [this comment](https://github.com/alan-turing-institute/MLJ.jl/issues/661#issue-705225473) uses the deprecated `@from_network` macro. See [these docs](https://alan-turing-institute.github.io/MLJ.jl/dev/learning_networks/#Exporting-a-learning-network-as-a-new-model-type) for the proper way to export learning networks.

