Update install from source instructions

From go 1.18 onwards (https://tip.golang.org/doc/go1.18#tools), to install a package from source, we have to use `go install`.
Since the command is already available before (from 1.16 onwards) and it's the recommended way to install a package, why not use it?

README.md

@@ -139,7 +139,7 @@ In case there isn't an official package for your operating system or architectur
 
 k6 is written in Go, so it's just a single statically-linked executable and very easy to build and distribute. To build from source you need **[Git](https://git-scm.com/downloads)** and **[Go](https://golang.org/doc/install)** (1.16 or newer). Follow these instructions:
 
-- Run `go get go.k6.io/k6` which will:
+- Run `go install go.k6.io/k6` which will:
   - git clone the repo and put the source in `$GOPATH/src/go.k6.io/k6`
   - build a `k6` binary and put it in `$GOPATH/bin`
 - Make sure you have `$GOPATH/bin` in your `PATH` (or copy the `k6` binary somewhere in your `PATH`), so you are able to run k6 from any location.