Convert static method variable to private class property in Countable::count() example.

---
Provided by online editor anonymous user 34863

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331901 c90b9560-bf6c-de11-be94-00142212c4b1
#65902 clarify wording

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331902 c90b9560-bf6c-de11-be94-00142212c4b1