[ticket/12196] Fix typo "referer" in acp/attachments

PHPBB3-12196
Merge remote-tracking branch 'nickvergessen/ticket/12196' into develop-olympus

# By Joas Schilling
# Via Joas Schilling
* nickvergessen/ticket/12196:
  [ticket/12196] Fix typo "referer" in acp/attachments