make : add train-text-from-scratch (#1850)

* make finetuning example accessible

* fixed: targed was in wrong line

* fixed: name of executable was wrong

* fixed: naming of binary

* fixed: model path was wrong

* fixed clean target

* Update examples/train-text-from-scratch/README.md

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>