Fix CHANGELOG entry [ci skip]

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
-*   `CollectionAssociation#count` returns 0 without querying if the
-    parent record is new.
+*   `CollectionAssociation#count` returns `0` without querying if the
+    parent record is not persisted.
 
     Before:
 
-        person.pets
+        person.pets.count
         # SELECT COUNT(*) FROM "pets" WHERE "pets"."person_id" IS NULL
         # => 0
 
     After:
 
-        person.pets
+        person.pets.count
         # fires without sql query
         # => 0