Update lintian override to php-markdown

The php-markdown package is now recommended over the older
libmarkdown-php, but it's still a false positive for us.

debian/php-illuminate-mail.lintian-overrides

 # False positive
-embedded-php-library usr/share/php/Illuminate/Mail/Markdown.php please use libmarkdown-php
+embedded-php-library usr/share/php/Illuminate/Mail/Markdown.php please use php-markdown