Doc: update path to `@popperjs/core` in parcel.md

Created by: wxh06

 ├── node_modules/
 │   └── bootstrap/
-│   └── popper.js/
+│   └── @popperjs/
+|       └── core/

According to popper.js on npm, this package has been deprecated. Bootstrap is now using @popperjs/core instead.