rename file name to be consistent with rest

news_item.yml as seen above is an exception
because the word 'news' is not countable.
Hence the _item form instead of just singular.

docs/api/apiv3/openapi-spec.yml

@@ -204,7 +204,7 @@ paths:
   "/api/v3/notifications":
     "$ref": "./paths/notifications.yml"
   "/api/v3/notifications/{id}":
-    "$ref": "./paths/notifications_item.yml"
+    "$ref": "./paths/notification.yml"
   "/api/v3/posts/{id}":
     "$ref": "./paths/post.yml"
   "/api/v3/posts/{id}/attachments":