Super tiny fix of dead link (#128160)

... found when looking at leak tracker today.

By the way, shall we add some kind of automatic link linter, which goes through all links and see whether they are alive?

packages/flutter/test/foundation/leak_tracking.dart

@@ -81,7 +81,7 @@ bool _webWarningPrinted = false;
 /// The method will fail if wrapped code contains memory leaks.
 ///
 /// See details in documentation for `withLeakTracking` at
-/// https://github.com/dart-lang/leak_tracker/blob/main/lib/src/orchestration.dart#withLeakTracking
+/// https://github.com/dart-lang/leak_tracker/blob/main/lib/src/leak_tracking/orchestration.dart
 ///
 /// The Flutter related enhancements are:
 /// 1. Listens to [MemoryAllocations] events.