Fix Crossplane help link
What does this MR do?
Reported in !19675 (comment 249483444)
Fixes bad link for Crossplane in clusters application page
https://docs.gitlab.com/ee/user/clusters/applications.html#crossplane is the correct link
Does this MR meet the acceptance criteria?
- Changelog entry
- [-] Documentation (if required)
- Code review guidelines
- Merge request performance guidelines
- Style guides
- [-] Database guides
- [-] Separation of EE specific content