Update README deprecated "webpackMode" in favor of "fetch"

[kl3sk]

After @weaverryan symfony/stimulus-bridge#15 PR

Encore complains:
The "webpackMode" config key is deprecated in controllers.json. Use "fetch" instead, set to either "eager" or "lazy".

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony UX repository. It is used by Composer to allow developers to depend on specific Symfony UX packages.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/ux

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the main branch.