Fix json_decode Twig filter

[AugustMiller]

Looks like the docs for this were correct (that it called Json::decode(...)), but the actual method name was still just the raw PHP function name.

See the original PR: #3678

[brandonkelly]

Doh, thanks!

[boboldehampsink]

Ah, just wanted to create an issue. I'm seeing

yii\base\ErrorException: call_user_func_array() expects parameter 1 to be a valid callback, class 'craft\helpers\Json' does not have a method 'json_decode' in

But I guess this fixes it