Actually, a smarter version (and in keeping with the override ethos) is to use the following instead.
PHP Code:
<?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . ($products_quantity>=2?TEXT_PRODUCT_IN_STOCK:($products_quantity>=1?TEXT_PRODUCT_LIMITED_STOCK:TEXT_PRODUCT_ON_ORDER)). '</li>' : '') . "\n"; ?>
and copy/paste the following into includes/languages/english/YOUR_CUSTOM_FOLDER/product_info.php
PHP Code:
define('TEXT_PRODUCT_IN_STOCK', 'In stock');
define('TEXT_PRODUCT_LIMITED_STOCK', 'Limited stock');
define('TEXT_PRODUCT_ON_ORDER', 'On order');
(I pasted it into line 22). Otherwise I've attached the file so you can just store it to includes/languages/english/YOUR_CUSTOM_FOLDER/