MINOR: Fix error message in SnapshotWriter.java (#9862)

Reviewers: Chia-Ping Tsai <[email protected]>

Author: tang7526

raft/src/main/java/org/apache/kafka/snapshot/SnapshotWriter.java

@@ -107,7 +107,7 @@ public boolean isFrozen() {
     public void append(List<T> records) throws IOException {
         if (snapshot.isFrozen()) {
             String message = String.format(
-                "Append not supported. Snapshot is already frozen: id = {}.",
+                "Append not supported. Snapshot is already frozen: id = '%s'.",
                 snapshot.snapshotId()
             );