docs(README): Fix path to nouislider CSS

[tony]

Fixes #48

With nouislider 15.6.1, the path will be dist/

❯ ls /node_modules/nouislider/
LICENSE.md  README.md  dist  package.json  src
 ls /node_modules/nouislider/dist
nouislider.css  nouislider.d.ts  nouislider.js  nouislider.min.css  nouislider.min.js  nouislider.min.mjs  nouislider.mjs
❯ cat node_modules/nouislider/package.json
{
  "name": "nouislider",
  "version": "15.6.1",
  ...
}