Update DOI badge link in README.md

[harryswift01]

Summary

This PR updates the DOI link in README.md to use Zenodo’s concept DOI for aiida-amber, ensuring it always resolves to the latest release rather than a specific archived version.

Changes

README.md

• Updated the Zenodo DOI from 18400804 (version-specific DOI) to 18400803 (concept DOI / persistent identifier).

Impact

• Ensures that each new aiida-amber release is automatically referenced correctly from the README.md.
• Prevents the documentation from becoming outdated by always linking to the current published version on Zenodo.

[coveralls]

Pull Request Test Coverage Report for Build 21515640031

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.949%

---

Totals
Change from base Build 21509679251:	0.0%
Covered Lines:	713
Relevant Lines:	1252

---

💛 - Coveralls

[jimboid]

Thanks for doing this