Fix isAfter method code example on DateTime class
Fix isAfter method code example on DateTime class

The code example looks similar as [isBefore method](https://github.com/dart-lang/sdk/blob/b8d040e5239e02b945b529347d0b980fc8a26db2/sdk/lib/core/date_time.dart#L405-L417) except in this change.

Closes https://github.com/dart-lang/sdk/pull/46847
https://github.com/dart-lang/sdk/pull/46847

GitOrigin-RevId: 4087032bcbd64e8aaf58dab81babb971d4642c86
Change-Id: I0b01d82ce4fad9ae076c4a1d46ee03cbc3b08d25
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209460
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk