docs(ja): use English operator names in choose-index.md

[yahonda]

Replaced Japanese katakana operator names with their original English names since these are proper operator names in TiDB:

Before	After
テーブルリーダー	TableReader
インデックスリーダー	IndexReader
インデックスマージ	IndexMerge

Also fixed: 列ベースのstorage → 列ベースのストレージ

Link texts and descriptions remain in Japanese.

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[gemini-code-assist]

Code Review

This pull request updates the choose-index.md Japanese documentation by replacing translated operator names (such as "テーブルリーダー", "インデックスリーダー", and "インデックスマージ") with their English equivalents ("TableReader", "IndexReader", and "IndexMerge") to maintain consistent terminology. The reviewer pointed out that one of the markdown links was changed to English ([Explain Statements Using Index Merge]), which contradicts the PR description stating that link texts should remain in Japanese. A suggestion was provided to keep the link text in Japanese as [インデックスマージを使用したステートメントの説明].

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