Update list packages per app API description (#4459)

In all "list" endpoint example responses there is a common pattern:
```http
HTTP/1.1 200 OK
Content-Type: application/json

<%= yield_content :paginated_list_of_[resource] %>
```

This pattern is broken in this case and makes it inconsistent compared to the rest of the documentation

Author: rrashidov

docs/v3/source/includes/resources/packages/_list_for_app.md.erb

@@ -16,8 +16,8 @@ Example Response
 
 ```http
 HTTP/1.1 200 OK
-```
-```json
+Content-Type: application/json
+
 <%= yield_content :paginated_list_of_associated_packages %>
 ```