[onert] Add space to increase readability

This commit adds space to increase readability.

ONE-DCO-1.0-Signed-off-by: Jiyoung Yun <[email protected]>

runtime/onert/core/include/ir/train/operation/UntrainableOperation.h

@@ -51,7 +51,7 @@ class UntrainableOperation : public OperationType, public TrainableOperation
   void accept(OperationVisitor &v) const override { v.visit(*this); }
   void accept(TrainableOperationVisitor &) const override
   {
-    throw std::runtime_error(OperationType::name() + "operation is not trainable yet");
+    throw std::runtime_error(OperationType::name() + " operation is not trainable yet");
   }
   bool hasTrainableParameter() const override { return false; }
 };