Fix doc links for adding a cluster to a project

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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

 - more_info_link = link_to _('More information'), help_page_path('user/project/clusters/index.md',
-  anchor: 'adding-an-existing-kubernetes-cluster'), target: '_blank'
+  anchor: 'add-existing-kubernetes-cluster'), target: '_blank'
 - rbac_help_link = link_to _('More information'), help_page_path('user/project/clusters/index.md',
   anchor: 'role-based-access-control-rbac-core-only'), target: '_blank'
 

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/index', anchor: 'adding-an-existing-kubernetes-cluster'), target: '_blank', rel: 'noopener noreferrer')
+  - link_to_help_page = link_to(s_('ClusterIntegration|documentation'), help_page_path('user/project/clusters/index', anchor: 'add-existing-kubernetes-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 }