Fix broken link to add_remove_clusters

app/views/clusters/clusters/user/_header.html.haml

 %h4
   = s_('ClusterIntegration|Enter the details for your Kubernetes cluster')
 %p
-  - link_to_help_page = link_to(s_('ClusterIntegration|documentation'), help_page_path('user/project/clusters/add_remove_cluster', anchor: 'add-existing-cluster'), target: '_blank', rel: 'noopener noreferrer')
+  - link_to_help_page = link_to(s_('ClusterIntegration|documentation'), help_page_path('user/project/clusters/add_remove_clusters', anchor: 'add-existing-cluster'), target: '_blank', rel: 'noopener noreferrer')
   = s_('ClusterIntegration|Please enter access information for your Kubernetes cluster. If you need help, you can read our %{link_to_help_page} on Kubernetes').html_safe % { link_to_help_page: link_to_help_page }