“Leave a Reply” comment text not changing

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I’m trying to change “leave a reply” to “Share a Comment”. I saw on multiple websites to go into comments.php and put in this code:

<?php comment_form(array('title_reply'=>'Share a Comment')); ?>

It’s not working. I also tried editing comment-template.php but I was told not to change core file themes.

What else can I do?

Website: http://jennifersweete.com/

Thanks!