tools/benchmark/README.md: Update installation as go get does not work
`etcd/tools/benchmark` is the official benchmarking tool for etcd clusters.
-## Download and install
-To get `benchmark` from the `main` branch via `go get`:
+## Install
+
+To install `benchmark` follow these steps.
+
```sh
-$ go get go.etcd.io/etcd/tools/benchmark
-# GOPATH should be set
-$ ls $GOPATH/bin
-benchmark
+$ go get go.etcd.io/etcd/v3/tools/benchmark
+$ benchmark --help
```
+
+Last command should output of usage of `benchmark`.
