Fix sign error in expiration calculation in MOQT.

PiperOrigin-RevId: 810995740
diff --git a/quiche/quic/moqt/moqt_relay_track_publisher.cc b/quiche/quic/moqt/moqt_relay_track_publisher.cc
index 9c0cddc..73c61a1 100644
--- a/quiche/quic/moqt/moqt_relay_track_publisher.cc
+++ b/quiche/quic/moqt/moqt_relay_track_publisher.cc
@@ -281,11 +281,12 @@
   if (expiration_ == quic::QuicTime::Infinite()) {
     return quic::QuicTimeDelta::Infinite();
   }
-  if (expiration_ < clock_->Now()) {
+  quic::QuicTime now = clock_->Now();
+  if (expiration_ < now) {
     // TODO(martinduke): Tear everything down; the track is expired.
     return quic::QuicTimeDelta::Zero();
   }
-  return clock_->Now() - *expiration_;
+  return *expiration_ - now;
 }
 
 void MoqtRelayTrackPublisher::DeleteTrack() {