docs: fix remaining language errors

[loama]

What

Fix eight objective spelling errors in the published NeuralForecast documentation and model parameter text.

Why

These mistakes are visible on the generated Nixtlaverse pages. The corrections preserve the technical meaning and change only misspelled words.

How

1. Correct three Prerequisites headings.
2. Correct the descriptions for cutoff timestamps and RevIn.
3. Correct spelling in the TFT and TimesNet model pages.

Testing

1. Parsed all four 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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