Update intro sentence

Author: mtarld

service_container/service_decoration.rst

@@ -316,9 +316,8 @@ The generated code will be the following::
 Control the Behavior When the Decorated Service Does Not Exist
 --------------------------------------------------------------
 
-Let's imagine you have a decorator that decorates a service defined by another bundle.
-If for some reasons, that bundle is missing, the ``decoration_on_invalid``
-option will allow you to handle the lack of the decorated service.
+When you decorate a service that doesn't exists, the ``decoration_on_invalid``
+option allows you to choose the behavior to adopt.
 
 .. configuration-block::