docs: Change link to label-studio-sdk/converter in README.md

[strickvl]

Removed the reference to the conversion tool as it's been merged into the sdk repo / functionality.

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
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• Docs have been added/updated (for bug fixes/features)
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• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

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• Product design
• Backend (Database)
• Backend (API)
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What does this fix?

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What is the new behavior?

(if this is a breaking or feature change)

What is the current behavior?

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Does this change affect performance?

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[robot-ci-heartex]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[strickvl]

Please don't remove this. Still applies, I think...