fix url in keras docs (#2134)
fix url in keras docs (#2134)


zhangxiaoli73/BigDL,i8run/BigDL-1,zhangxiaoli73/BigDL,qiuxin2012/BigDL,wzhongyuan/BigDL,i8run/BigDL-1,wzhongyuan/BigDL,yiheng/BigDL,qiuxin2012/BigDL,i8run/BigDL-1,zhangxiaoli73/BigDL,qiuxin2012/BigDL,yiheng/BigDL,wzhongyuan/BigDL,yiheng/BigDL