fixed 404 for Edit this page ilnk on upstream repo topics

Signed-off-by: craig-osterhout <craig.osterhout@docker.com>

_config_production.yml

@@ -79,6 +79,10 @@ defaults:
       path: engine/extend.md
     values:
       edit_url: "https://github.com/docker/cli/tree/master/docs/extend"
+  - scope:
+      path: engine/extend
+    values:
+      edit_url: "https://github.com/docker/cli/tree/master/docs/extend"
   - scope:
       path: engine/reference
     values: