minor #65 Update README deprecated "webpackMode" in favor of "fetch" (kl3sk)

This PR was merged into the main branch.

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

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".`

| 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
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463f429 Update README.md

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
                 }