Update used-by.md

Hey there! ActiveCollab is the project management tool operating since 2007.
ActiveCollab's mobile team strongly rely on React Navigation.
We would be very happy if you could list us on this page.

Thank you in advance! :)

Author: BojanRatkovic

versioned_docs/version-6.x/used-by.md

@@ -46,6 +46,7 @@ It's impossible to list every single app that uses React Navigation, but below a
 - [Tracker Network for Fortnite](https://apps.apple.com/us/app/tracker-network-for-fortnite/id1287696482)
 - [vrbo](https://www.vrbo.com/mobile/)
 - [Disprz](https://www.disprz.com/) ([iOS](https://apps.apple.com/us/app/disprz/id1458716803#?platform=iphone)) ([Android](https://play.google.com/store/apps/details?id=com.disprz&hl=en_IN&gl=US))
+- [ActiveCollab](https://activecollab.com/) ([iOS](https://apps.apple.com/us/app/activecollab-work-management/id1509421965)) ([Android](https://play.google.com/store/apps/details?id=com.activecollab.mobile))
 
 ## Your app?