Use correct variable in WP_Automatic_Updater::send_email().

Merges [25977] to the 3.7 branch and adds an inline comment.

props pento.
see #25757.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@25981 602fd350-edb4-49c9-b593-d223f7449a82
3.7.1


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@25982 602fd350-edb4-49c9-b593-d223f7449a82