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 #46847
#46847

GitOrigin-RevId: 4087032
Change-Id: I0b01d82ce4fad9ae076c4a1d46ee03cbc3b08d25
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209460
Reviewed-by: Lasse R.H. Nielsen <[email protected]>
Commit-Queue: Lasse R.H. Nielsen <[email protected]>

Author: niku

sdk/lib/core/date_time.dart

@@ -432,8 +432,8 @@ class DateTime implements Comparable<DateTime> {
   /// assert(later.isAfter(now.toUtc()));
   /// assert(later.toUtc().isAfter(now));
   ///
-  /// assert(!now.toUtc().isBefore(now));
-  /// assert(!now.isBefore(now.toUtc()));
+  /// assert(!now.toUtc().isAfter(now));
+  /// assert(!now.isAfter(now.toUtc()));
   /// ```
   external bool isAfter(DateTime other);