🌐 [i18n-KO] Fixed `tutorial/preprocessing.mdx`
Author: sim-so
Pull request: yes

# What does this PR do?

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Fixed some words and expressions to align with the latest translation works.
This PR is a revision to apply key terms and phrases that have been established over the course of translating several documents. 

Here are the main changes:
- `dataset` : `데이터셋` -> `데이터 세트`
- `truncation` : `생략` -> `잘라내기`
- `train` : `학습` -> `훈련`

Some other sentences have also been modified to read more naturally.

Thank you in advance for your review!

Fixes #22578 
Part of https://github.com/huggingface/transformers/issues/20179

## Before reviewing
- [x] Check for missing / redundant translations (번역 누락/중복 검사)
- [x] Grammar Check (맞춤법 검사)
- [x] Review or Add new terms to glossary (용어 확인 및 추가)
- [x] Check Inline TOC (e.g. `[[lowercased-header]]`)
- [x] Check live-preview for gotchas (live-preview로 정상작동 확인)

## Who can review? (Initial)
Team PseudoLab, may you please review this PR? 
@0525hhgus, @KIHOON71, @sim-so, @gabrielwithappy, @HanNayeoniee, @wonhyeongseo, @jungnerd

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
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      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

May you please review this PR?
@sgugger, @ArthurZucker, @eunseojo 

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Comments:
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