[Docs] Problem with getting-started/explaining-globs

As discovered in https://github.com/gulpjs/gulp/issues/2275#issuecomment-469073132, there's an issue in node-glob where a directory passed to ignore (like our !node_modules/ example) doesn't actually exclude the folder from globbing. I'm not sure if this is a regression in node-glob but we should remove the example from the guide.