[Apps SDK] Fix rich app link in documentation (#19007)

- Fix link to rich app for LLMS
- Add example of extension of existing object in rich app (post card
app)

packages/create-twenty-app/src/constants/base-application/LLMS.md

@@ -1,7 +1,7 @@
 ## Base documentation
 
 - Documentation: https://docs.twenty.com/developers/extend/capabilities/apps
-- Rich app example: https://github.com/twentyhq/twenty/tree/main/packages/twenty-sdk/src/app-seeds/rich-app
+- Rich app example: https://github.com/twentyhq/twenty/tree/main/packages/twenty-apps/fixtures/postcard-app
 
 ## UUID requirement