docs: fix remaining language errors

[loama]

What

Fix two objective spelling errors in the published StatsForecast documentation.

Why

The current experiment and tutorial pages contain misspelled verbs. The corrections preserve the technical meaning.

How

1. Change substract to subtract.
2. Change specifices to specifies.

Testing

1. Parsed both modified notebooks as valid JSON.
2. Confirmed the reported misspellings are absent from the affected files.
3. Reviewed the complete diff.
4. Confirmed no secrets, credentials, or generated binaries are included.

Checklist

1. The title follows the conventional commit format.
2. The pull request is focused on one concern.
3. The complete diff was reviewed.
4. No secrets, credentials, or personal data are included.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 38 untouched benchmarks

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_{Comparing fix/remaining-language-errors (559779d) with main (a580cf9)}