[FrameworkBundle] fix changelog

The `framework.serializer.cache` option was also deprecated in Symfony
3.1. This was documented in the upgrade file, but was not reflected in
the bundle's changelog.

src/Symfony/Bundle/FrameworkBundle/CHANGELOG.md

@@ -21,7 +21,8 @@ CHANGELOG
  * Deprecated using core form types without dependencies as services
  * Added `Symfony\Component\HttpHernel\DataCollector\RequestDataCollector::onKernelResponse()`
  * Added `Symfony\Bundle\FrameworkBundle\DataCollector\RequestDataCollector`
- * Deprecated service `serializer.mapping.cache.apc` (use `serializer.mapping.cache.doctrine.apc` instead)
+ * The `framework.serializer.cache` option and the service `serializer.mapping.cache.apc` have been
+   deprecated. APCu should now be automatically used when available.
 
 3.0.0
 -----