fix missing blank line in doc (#2451)
fix missing blank line in doc (#2451)


yiheng/BigDL,zhangxiaoli73/BigDL,wzhongyuan/BigDL,wzhongyuan/BigDL,i8run/BigDL-1,yiheng/BigDL,i8run/BigDL-1,zhangxiaoli73/BigDL,wzhongyuan/BigDL,zhangxiaoli73/BigDL,yiheng/BigDL,i8run/BigDL-1