Fixing broken link to experiment cleanup doc

What does this MR do and why?

Fixes broken link to Experiment Cleanup in documentation.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://docs.gitlab.com/development/experiment_guide/

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit https://docs.gitlab.com/development/experiment_guide
2. Click Link for "Experiment Cleanup"

(Or verify by visiting links in the diff)