Update docs/machine_learning_tutorial.md
Update docs/machine_learning_tutorial.md

Co-Authored-By: Noah Fahlgren <273076a8b6ff7a0ef6e9aa218c719e92c1be6226@gmail.com>
danforthcenter/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,stiphyMT/plantcv,stiphyMT/plantcv,danforthcenter/plantcv