message: Fix example in comments for CancellationManager::RegisterCallback
diff: @@ -79,7 +79,7 @@ class CancellationManager {
  // CancellationToken token = cm->get_cancellation_token();
  // {
  // mutex_lock(mu_);
  - // already_cancelled = cm->RegisterCallback(
  + // already_cancelled = !cm->RegisterCallback(
  // [this, token]() { Cancel(token); });
  // if (!already_cancelled) {
  // // Issue asynchronous operation. Associate the pending operation
  