Fix sed command in DEVELOPER_GUIDE to append a new line character '\n…

[0ctopus13prime]

Description

Back porting https://github.com/opensearch-project/k-NN/pull/2181 to 2.x branch.

Related Issues

N/A

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [ ] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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