README.md: replace `get -u` with `install`

[Hi-Angel]

Running go get -u outside repo complains:

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'.

Running it inside the repo simply updates clipboard module but does not install the 2fa.

So use a go install instead.

[Hi-Angel]

Ping. My colleague is configuring your utility, and the issue still remains. Any news on this PR?

[Hi-Angel]

Ping