Documentation: use Go 1.9+ in dl_build.md
Documentation: use Go 1.9+ in dl_build.md

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

akauppi/etcd,joshix/etcd,visheshnp/etcd,glevand/coreos--etcd,dannysauer/etcd,bdarnell/etcd,sebrandon1/etcd,etcd-io/etcd,akauppi/etcd,gyuho/etcd,joshix/etcd,cockroachdb/etcd,lorneli/etcd,mqliang/etcd,ajityagaty/etcd,mkumatag/etcd,xiang90/etcd,rnaveiras/etcd,mqliang/etcd,bluebreezecf/etcd,ajityagaty/etcd,cockroachdb/etcd,bdarnell/etcd,gyuho/etcd,lorneli/etcd,etcd-io/etcd,xiang90/etcd,etcd-io/etcd,ajityagaty/etcd,heyitsanthony/etcd,heyitsanthony/etcd,glevand/coreos--etcd,whuwxl/etcd,bluebreezecf/etcd,fanminshi/etcd,li-ang/etcd,visheshnp/etcd,mqliang/etcd,vimalk78/etcd,sinsharat/etcd,vimalk78/etcd,whuwxl/etcd,rnaveiras/etcd,YuleiXiao/etcd,sebrandon1/etcd,sinsharat/etcd,YuleiXiao/etcd,mkumatag/etcd,rnaveiras/etcd,dannysauer/etcd,li-ang/etcd,coreos/etcd,mkumatag/etcd,fanminshi/etcd,coreos/etcd