fix a typo in rank_loss_op

paddle/operators/rank_loss_op.cc

@@ -39,7 +39,7 @@ class RankLossOp : public framework::OperatorWithKernel {
     PADDLE_ENFORCE((label_dims == left_dims) && (left_dims == right_dims),
                    "All inputs must have the same size");
     PADDLE_ENFORCE((label_dims.size() == 2) && (label_dims[1] == 1),
-                   "All inputs must be row vector with size batch_sizex1.");
+                   "All inputs must be row vector with size batch_size x 1.");
     ctx.Output<framework::LoDTensor>("Out")->Resize(label_dims);
   }
 };