update metadata from ms.prod to ms.service (SharePoint#7930)

Author: huypub

docs/docfx.json

@@ -42,7 +42,7 @@
       "author": "VesaJuvonen",
       "ms.topic": "conceptual",
       "ms.suite": "office",
-      "ms.prod": "sharepoint",
+      "ms.service": "sharepoint",
       "feedback_system": "GitHub",
       "feedback_github_repo": "SharePoint/sp-dev-docs",
       "feedback_product_url": "https://aka.ms/spdev-uservoice",