The language strings are kept in a cache on the server. You will need to purge the cache to see the new string
Go to
/admin/purgecaches.php
Although it would be much better to change the string via language customisation rather than the code. Then when you upgrade Moodle, any language changes you make will still be there
/admin/tool/customlang/index.php
EDIT: Forgot to mention - during testing, you can switch off the language cache in config.php
$CFG->langstringcache = false;