Update reference.dart

[mafreud]

This pull request includes changes to the Query class in the packages/dart_firebase_admin/lib/src/google_cloud_firestore/reference.dart file. The changes modify the way the orderBy method is used in the examples provided in the comments. Instead of using a string to indicate the sort order, a named boolean parameter descending is now used.

Main changes:

• packages/dart_firebase_admin/lib/src/google_cloud_firestore/reference.dart: In the Query class, the orderBy method in the examples has been changed from using a string parameter to indicate the sort order to using a named boolean parameter descending. This change has been applied in two places in the comments. [1] [2]