fixed typo in callbacks.proto

Author: graham63

src/proto/callbacks.proto

@@ -86,7 +86,8 @@ message Callback {
     CallbackPerturbWeights perturb_weights = 52;
     CallbackExportOnnx export_onnx = 53;
     CallbackAlternateUpdates alternate_updates = 54;
-    CallbackMlperfLogging mlperf_logging = 55;
+    CallbackMlperfLogging mlperf_logging = 56;
+}
 
   message CallbackLTFB {
     int64 batch_interval = 1;