README.md: correct installation instructions
README.md: correct installation instructions

Fixes golang/go#28196

Change-Id: I5fa57b039798c172a0b2610d8278077aca15512e
Reviewed-on: https://go-review.googlesource.com/c/141802
Reviewed-by: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/review