Fix assert! macro usage

There is a bug in rustc that allows adding invalid trailing tokens to
the `assert!` macro call. They are currently ignored but are going to
produce errors in the future.

Fix assert! macro usage to add missing comma.

For more information, see
rust-lang/rust#60024 and
rust-lang/rust#60039

core/src/recovery.rs

@@ -208,7 +208,7 @@ impl QuicRecovery {
         };
 
         assert!(
-            self.largest_sent_packet < seq
+            self.largest_sent_packet < seq,
             "cannot send packet older than last one"
         );
         self.largest_sent_packet = seq;