docs: restore 1 dead link(s) via Wayback

[mirkosalvato1-ctrl]

This PR fixes 1 broken outbound documentation link(s).

Changes:

• README.md: https://artifacts-api.elastic.co/v1/search/8.0-SNAPSHOT/ → http://web.archive.org/web/20210918005231/https://artifacts-api.elasti (Wayback snapshot (host dead))

Fixes #50539

[botelastic]

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[cla-checker-service]

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[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /test : Run the Buildkite pipeline.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @mirkosalvato1-ctrl? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.