fix error in admin_dledit.php

When editing a download where some files are marked as mirror,
the mirror checkbox will not be checked due to incorrect HTML
syntax for the input element (caused by a typo). As a result
all "mirror" files in the edited download will be saved as
non-mirror files with the next update - unless the mirror flag
is set manually by the person editing the download.

The commit fixes that bug. It also fixes some minor spelling
mistakes in the script.