Fix command to install vultr-cli using Go

[zikalino]

Description

Command:

    go get -u github.com/vultr/vultr-cli/v3

doesn't seem to work any longer, it returns:

go: go.mod file not found in current directory or any parent directory.
'go get' is no longer supported outside a module.
To build and install a command, use 'go install' with a version,
like 'go install example.com/cmd@latest'
For more information, see https://golang.org/doc/go-get-install-deprecation
or run 'go help get' or 'go help install'.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[optik-aper]

Looks good! Thanks for the PR