doc: Use stable version constraint for sylius-search-plugin

Changed the composer require command in the README file to suggest using the stable version of the monsieurbiz/sylius-search-plugin.

Author: panakour

README.md

@@ -13,7 +13,7 @@ A search plugin for Sylius using [Elastically](https://github.com/jolicode/elast
 
 Require the plugin :
 ```
-composer require monsieurbiz/sylius-search-plugin="^2.0@RC"
+composer require monsieurbiz/sylius-search-plugin="^2"
 ```
 
 If you are using Symfony Flex, the recipe will automatically do some actions.