[perf_witness] Fix import statement

For some reason both CFE and Analyzer don't seem to complain about
../../server.dart and instead resolve it as if it was ../server.dart.

Issue #62233

Change-Id: I5a4ca8c7c6686c08d06e709bacfce2c96a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467800
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>

Author: mraleph

pkg/perf_witness/lib/src/async_span.dart

@@ -5,7 +5,7 @@
 import 'dart:async';
 import 'dart:developer';
 
-import '../../server.dart';
+import '../server.dart';
 
 /// With synchronous execution the nesting between spans is naturally induced
 /// by the callstack. Consider: