[ticket/11313] Use correct object el instead of eel in alt_text callback

This typo was added by the PRs #860 and #1178. Only the object el exists
and is correct. Due to this the text of links using the alt_text callback
were not modified while executing the callback.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11313

PHPBB3-11313