docs: fix composer config to avoid modifying package.json automatically

Author: Crovitche-1623

src/Dropzone/assets/README.md

@@ -14,7 +14,7 @@ composer require symfony/ux-dropzone:2.23.0
 npm add @symfony/[email protected]
 ```
 
-**Tip:** Your `package.json` file will be automatically modified by [Flex](https://github.com/symfony/flex) when installing or upgrading a PHP package. To prevent this behavior, ensure to **use at least Flex 1.22.0 or 2.5.0**, and run `composer config extra.symfony.flex.synchronize_package_json false`.
+**Tip:** Your `package.json` file will be automatically modified by [Flex](https://github.com/symfony/flex) when installing or upgrading a PHP package. To prevent this behavior, ensure to **use at least Flex 1.22.0 or 2.5.0**, and run `composer config "extra.symfony/flex.synchronize_package_json" false`.
 
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