message: Update model_fn.py
diff: @@ -219,7 +219,7 @@ class ModelFnOps(
  used if a Servo request does not explicitly mention which head to infer
  on. Pass the key of the output alternative here that you want to
  designate as default. A separate ExportOutpout for this default head
  - wil be added to the export_outputs dict with the special key
  + will be added to the export_outputs dict with the special key
  signature_constants.DEFAULT_SERVING_SIGNATURE_DEF_KEY, unless there is
  already an enry in output_alternatives with this special key.
  