Update firestore-tos-extension/POSTINSTALL.md
Update firestore-tos-extension/POSTINSTALL.md

Co-authored-by: Elliot Hesp <9e878c36314a3f94192fcb3b149b1da50c21480d@gmail.com>
FirebaseExtended/experimental-extensions,FirebaseExtended/experimental-extensions,FirebaseExtended/experimental-extensions