Limit word plugin

I am using the word limit plugin (https://labitacora.net/index.php?p=152). One of the instructions says:

4. Go into your index.php file and change the function the_content() with the_content_limit($max_characters, $link_text) where:
$max_characters: maximum amount of characters shown for an entry in the main page (index.php)
$link_text: text that is used to generate the link to the whole article.

When I open index.php all I can see is:

<?php
/* Short and sweet */
define(‘WP_USE_THEMES’, true);
require(‘./wp-blog-header.php’);
?>

What do I do?