Fix incorrect call to json_encode()

Summary: I confused this with the signature of json_decode() when I wrote this.
PHP 5.2 json_encode() takes only one parameter, and PHP 5.3 json_encode() takes
a flags bitmask, so this call is strictly incorrect on all versions of PHP.
Test Plan: scrutinized http://us.php.net/manual/en/function.json-encode.php
Reviewed By: jungejason
Reviewers: davidreuss, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 573