minor #212 [ux/dropzone] Updated docs (deprecation warning) (evertharmeling)

This PR was merged into the 2.x branch.

Discussion
----------

[ux/dropzone] Updated docs (deprecation warning)

Fixed following warning:
```
Module Warning (from ./node_modules/`@symfony`/stimulus-bridge/dist/webpack/loader.js):
The "webpackMode" config key is deprecated in controllers.json. Use "fetch" instead, set to either "eager" or "lazy".
```

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Tickets       | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Commits
-------

96f2798 [ux/dropzone] Updated docs (deprecation warning)

Author: weaverryan

src/Dropzone/README.md

@@ -61,7 +61,7 @@ the `@symfony/ux-dropzone/src/style.css` autoimport to `false`:
         "@symfony/ux-dropzone": {
             "dropzone": {
                 "enabled": true,
-                "webpackMode": "eager",
+                "fetch": "eager",
                 "autoimport": {
                     "@symfony/ux-dropzone/src/style.css": false
                 }