your functions.php of the child theme should only have:
<?php
add_filter( 'the_content_more_link', 'modify_read_more_link' );
function modify_read_more_link() {
return '<a href="' . get_permalink() . '">Plus</a>';
?>[Moderator Note: Please post code & markup between backticks (not single quotes) or use the code button. Your posted code may now have been damaged by the forum's parser.]
without those backticks ('), of course.