[bug fix] cannot run webpack command

[sneko]

When taking the develop branch I'm not able to do npm run dev after npm install. Because it says:

> [email protected] dev
> webpack serve --progress --mode=development

[webpack-cli] Invalid options object. Dev Server has been initialized using an options object that does not match the API schema.
 - options.proxy should be an array:
   [object { … } | function, ...]
   -> Allows to proxy requests, can be useful when you have a separate API backend development server and you want to send API requests on the same domain.
   -> Read more at https://webpack.js.org/configuration/dev-server/#devserverproxy

With my modification it works.

[ps-jarvis]

Hello @sneko!

This is your first pull request on hummingbird repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!