Fix command in xgboost example readme (#5202)
Fix command in xgboost example readme (#5202)

Signed-off-by: Junwen Yao <143c29295ae14a652569f8671334194d952ca4eb@gmail.com>
mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow