Update RayOnSpark doc (#3242)
Update RayOnSpark doc (#3242)

* update doc

* update ray dependency

* meet review
intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL