benchmark: fix install docs
`etcd/tools/benchmark` is the official benchmarking tool for etcd clusters.
-## Install
-
-```bash
-go get github.com/coreos/etcd/tools/benchmark
+## Download and install
+To get `benchmark` from the `master` branch via `go get`:
+```sh
+$ go get go.etcd.io/etcd/tools/benchmark
+# GOPATH should be set
+$ ls $GOPATH/bin
+benchmark
```
