fix: Use correct string formatting for log message (#668)

Author: dpcollins-google

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/PartitionCountWatchingPublisher.java

@@ -184,7 +184,7 @@ private void handleConfig(long partitionCount) {
       }
       if (partitionCount < currentSize) {
         log.atWarning().log(
-            "Received an unexpected decrease in partition count. Previous partition count {}, new count {}",
+            "Received an unexpected decrease in partition count. Previous partition count %s, new count %s",
             currentSize,
             partitionCount);
         return;