README.md: don't recommend 'go get'

[ebiggers]

With the latest version of go, the recommended 'go get' commands fail:

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'.

Replace them with commands that work.