docs: Update Atsign's project using link

We've since deprecated at_app and archived it. at_mono is a much more niche / interesting use-case (cross-repo dependency analysis and testing). We wrote an explainer here: https://github.com/atsign-foundation/.github/blob/trunk/docs/at_mono.md

docs/index.mdx

@@ -49,7 +49,7 @@ The following projects are using Melos:
 - [ApptiveGrid/apptive_grid_flutter](https://github.com/ApptiveGrid/apptive_grid_flutter)
 - [flutternetwork/WiFiFlutter](https://github.com/flutternetwork/WiFiFlutter)
 - [iapicca/yak_packages](https://github.com/iapicca/yak_packages)
-- [atsign-foundation/at_app](https://github.com/atsign-foundation/at_app)
+- [atsign-foundation/at_mono](https://github.com/atsign-foundation/at_mono)
 - [sub6resources/flutter_html](https://github.com/sub6resources/flutter_html)
 - [ferraridamiano/ConverterNOW](https://github.com/ferraridamiano/ConverterNOW)
 - [rrifafauzikomara/youtube_video](https://github.com/rrifafauzikomara/youtube_video)