clientv3: fix indentation in doc.go
clientv3: fix indentation in doc.go

Looks off in https://godoc.org/github.com/coreos/etcd/clientv3.

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

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