This can be easily modified in the source code. Open /wp-includes/comment-template.php file and search for ‘Leave a Reply’ term. It will be in $default array-
//File: /wp-includes/comment-template.php
...
$default = array(
...
'title_reply' => __( 'Leave a Reply' ), // Modify here
'title_reply_to' => __( 'Leave a Reply to %s' ),
...
)