docs: Fix go install command in getting-started.md

[istvan-andrasi]

Summary

Advertised go install command does not work...

> go install github.com/asdf-vm/[email protected]
package github.com/asdf-vm/asdf: build constraints exclude all Go files in /home/isi/.cache/go/pkg/mod/github.com/asdf-vm/[email protected]

because entrypoint is under cmd/asdf. This works:

> go install github.com/asdf-vm/asdf/cmd/[email protected]
go: downloading github.com/sethvargo/go-envconfig v1.0.0
go: downloading github.com/go-git/go-git/v5 v5.13.0
go: downloading github.com/urfave/cli/v2 v2.27.1
go: downloading dario.cat/mergo v1.0.0
go: downloading github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376
go: downloading github.com/pjbgf/sha1cd v0.3.0
go: downloading github.com/ProtonMail/go-crypto v1.1.3
go: downloading github.com/go-git/go-billy/v5 v5.6.0
go: downloading github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99
go: downloading github.com/sergi/go-diff v1.3.2-0.20230802210424-5b0b94c5c0d3
go: downloading github.com/kevinburke/ssh_config v1.2.0
go: downloading github.com/skeema/knownhosts v1.3.0
go: downloading github.com/xanzy/ssh-agent v0.3.3
go: downloading gopkg.in/warnings.v0 v0.1.2
go: downloading github.com/cyphar/filepath-securejoin v0.2.5
go: downloading github.com/cloudflare/circl v1.3.7