fix typo in arguments of PartitionBuilder::Init (#7113)
fix typo in arguments of PartitionBuilder::Init (#7113)

Co-authored-by: Yuntian Zhang <2c19c2803d39c2da791f4fc739c40e033e5fb945@lamda.nju.edu.cn>
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost