Correct link to external_dashboard doc

app/views/projects/settings/operations/_external_dashboard.html.haml

 .js-operation-settings{ data: { operations_settings_endpoint: project_settings_operations_path(@project),
   external_dashboard: { url: metrics_external_dashboard_url,
-  help_page_path: help_page_path('user/project/operations/link_to_external_dashboard') } } }
+  help_page_path: help_page_path('user/project/operations/linking_to_an_external_dashboard') } } }

changelogs/unreleased/63691-fix-doc-link.yml

+---
+title: Correct link to docs for External Dashboard
+merge_request: 30019
+author:
+type: fixed