Fix typos in help links
What does this MR do?
Fix typos in help links as reported in !18233 (comment 240914086)
Bug
We are linking to https://gitlab.com/help/user/project/clusters/add_remove_cluster.md#add-existing-cluster
Fix
Fix is to link to https://gitlab.com/help/user/project/clusters/add_remove_clusters.md#add-existing-cluster
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
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