docs: fix duplicated "of" in machine-learning.md

[lphuc2250gma]

One-line typo fix in docs/reference/machine-learning.md: "This command results in a local copy of of the model in the directory bert-base-NER." → "This command results in a local copy of the model in the directory bert-base-NER.". No code/behavior change.

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[miguelgrinberg]

@lphuc2250gma I can't do anything with this PR until you sign the CLA. See above.