fix doc about max_depth (#4078)
fix doc about max_depth (#4078)

* fix doc

* Update doc/parameter.rst

Co-Authored-By: CodingCat <5c204eca2838d8caaedf1ad3d097d8d2dba65e8d@users.noreply.github.com>

dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost