Remove BC Break label from NullDumper class

symfony/dependency-injection@7f34aa2

This is the commit where the class has been made "final", but it has only been made final via php doc, not via code, so this shouldn't be a BC break in my opinion.

Author: kobelobster

UPGRADE-3.3.md

@@ -82,7 +82,7 @@ DependencyInjection
 
  * Autowiring services based on the types they implement is deprecated and won't be supported in version 4.0. Rename (or alias) your services to their FQCN id to make them autowirable.
 
- * [BC BREAK] The `NullDumper` class has been made final
+ * The `NullDumper` class has been made final
 
  * [BC BREAK] `_defaults` and `_instanceof` are now reserved service names in Yaml configurations. Please rename any services with that names.