Fix broken link to add_remove_clusters

What does this MR do?

Fix broken link to add_remove_clusters

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