v3rpc: fix a typo `err`
v3rpc: fix a typo `err`


don't read return value in child goroutine which causes data race.
dannysauer/etcd,ajityagaty/etcd,rnaveiras/etcd,rnaveiras/etcd,li-ang/etcd,mkumatag/etcd,etcd-io/etcd,cockroachdb/etcd,mqliang/etcd,etcd-io/etcd,gyuho/etcd,ajityagaty/etcd,mkumatag/etcd,li-ang/etcd,coreos/etcd,dannysauer/etcd,gyuho/etcd,ajityagaty/etcd,mqliang/etcd,mkumatag/etcd,coreos/etcd,cockroachdb/etcd,rnaveiras/etcd,mqliang/etcd,etcd-io/etcd