docs/getting-started/local.md: npm command line is now deprecated, use --location=global not -g

More recent npm (anything that'll work with Node-RED 3.x) has deprecated -g, so new users will get a benign warning. Might be worth applying sandpaper to that initial user experience :-).

I think they have changed / reverted that - npm/statusboard#518
so I'm going to leave it for a while - as long we still support node14 etc.