README: correct installation instructions

Fixes golang/go#23167

Change-Id: I04e9f92fd0fc83f1f489e9f2a328c4916c06b8e6
Reviewed-on: https://go-review.googlesource.com/84595
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
diff --git a/README.md b/README.md
index 2ba775d..4820db2 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@
 
 ## Download/Install
 
-The easiest way to install is to run `go get -u golang.org/x/perf`. You can
+The easiest way to install is to run `go get -u golang.org/x/perf/cmd/...`. You can
 also manually git clone the repository to `$GOPATH/src/golang.org/x/perf`.
 
 ## Report Issues / Send Patches