Fixed UPGRADE-4.0 about Container::set

Author: lyrixx

UPGRADE-4.0.md

@@ -116,8 +116,8 @@ DependencyInjection
 
  * Using unsupported options to configure service aliases raises an exception.
 
- * Setting or unsetting a private service with the `Container::set()` method is
-   no longer supported. Only public services can be set or unset.
+ * Setting or unsetting a service with the `Container::set()` method is
+   no longer supported. Only synthetic services can be set or unset.
 
  * Checking the existence of a private service with the `Container::has()`
    method is no longer supported and will return `false`.