chore: correct NDJSON extension for sample ecommerce data

[hainenber]

Description

Describe what this change achieves. The sample ecommerce data file is not JSON per se but actually is [NDJSON](https://github.com/ndjson/ndjson-spec?tab=readme-ov-file) as suggested by the accompanying Content-Type header.

This PR is to remediate it.

Issues Resolved

Closes #[insert issue number] N/A

Version

List the OpenSearch version to which this PR applies, e.g. 2.14, 2.12--2.14, or all. All

Frontend features

If you're submitting documentation for an OpenSearch Dashboards feature, add a video that shows how a user will interact with the UI step by step. A voiceover is optional. N/A

Checklist

• [x] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

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[kolchfa-aws]

@peterzhuamazon Could you please review this PR?

[kolchfa-aws]

@mgodwan Could you please review this PR?