change “missing attachment” text functions.php

You should never change a core file. This message is called by the function wp_get_attachment_link(). For this singular message this function does not apply a filter, but it return the string as value, which can help. You can simple identify where in your theme this function is called and change it return, like:

$result = wp_get_attachment_link();
if ($result == 'Missing Attachment')
  echo '-';
else
  echo $result;