[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.

PHPBB3-11313
Merge PR #1181 branch 'marc1706/ticket/11313' into develop

# By Marc Alexander
# Via Marc Alexander
* marc1706/ticket/11313:
  [ticket/11313] Use correct object el instead of eel in alt_text callback