Fix node_modules path

Created by: MartijnCuppens

On the getting started with theming page http://getbootstrap.com/docs/4.1/getting-started/theming/#importing, the path to the node_modules folder is wrong.

This is the file structure:

your-project/
├── scss
│   └── custom.scss
└── node_modules/
    └── bootstrap
        ├── js
        └── scss

So if we start from scss/custom.scss, we need to add ../ to get to the node_modules folder.