Fix README about BC promise

Author: nicolas-grekas

README.md

@@ -4,9 +4,10 @@ Messenger Component
 The Messenger component helps application send and receive messages to/from other applications or via
 message queues.
 
-**This Component is experimental**. [Experimental
-features](https://symfony.com/doc/current/contributing/code/experimental.html)
-are not covered by Symfony's BC-break policy.
+**This Component is experimental**.
+[Experimental features](https://symfony.com/doc/current/contributing/code/experimental.html)
+are not covered by Symfony's
+[Backward Compatibility Promise](https://symfony.com/doc/current/contributing/code/bc.html).
 
 Resources
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