Docs: Update link to example app (#18253)

The live website points to a github URL that no longer exists.

docs/sources/plugins/developing/apps.md

@@ -55,5 +55,5 @@ If possible a link to a dashboard or custom page should be shown after enabling
 
 > Our goal is not to have a very extensive documentation but rather have actual
 > code that people can look at. An example implementation of an app can be found
-> in this [example app repo](https://github.com/grafana/example-app)
+> in this [example app repo](https://github.com/grafana/grafana/tree/master/public/app/plugins/app/example-app)