_content/doc: update local tour installation instructions
_content/doc: update local tour installation instructions

We already use go install for the installation instructions in the
tour itself, but the command shown in the root doc document still uses
go get, which by itself won't install any binary in recent Go
versions. Switch to go install there too.

Fixes golang/go#48347

Change-Id: Iad6b22010c92ff0f60df47804fabdd3a3ac2d5c2
Reviewed-on: https://go-review.googlesource.com/c/website/+/349529
Trust: Alberto Donizetti <90b16103a35511ced020676f3544f17117fb4c92@gmail.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website