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?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers