Update action.md (#4949)

Fix path for action plugin docs

docs/docs/extend/plugins/action.md

@@ -15,4 +15,4 @@ POST {
 }
 ```
 
-For an example of a very simple action plugin, refer to `/InvenTree/plugin/builtin/action/simpleactionplugin.py`
+For an example of a very simple action plugin, refer to `/InvenTree/plugin/samples/integratoni/simpleactionplugin.py`