docs: Fix project export API parameter name to match route URL

[willeastcott]

Summary

• Rename projectId to id in the project export API parameter table and curl example to match the route URL pattern (POST .../projects/:id/export) and the convention used in other endpoint docs (app-get, job-get, checkpoint-get)
• Apply the same fix to the Japanese translation

Test plan

• Verify the parameter table shows id instead of projectId
• Verify the curl example uses {id} instead of {projectId}
• Verify the Japanese translation matches

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