npm usage should explain that it will open in a browser

This is really quite small, but it would be nice if the [usage output](https://github.com/npm/cli/blob/ab22c43a6a5ddcd9b59eb3df3db4e1a0dffdcaaa/lib/utils/npm-usage.js#L19) said that it would open in a browser. I forget every now and then and it surprises me. Maybe I'll submit a PR.