Bug#1094784: modernize-sources is missing from the apt(8) man page

I saw this too, but wasn't sure whether it was intentionally excluded as
part of the "only mentions the most used commands and options" threshold.

On the other hand, `apt --help` does list modernize-sources under a heading
of "Most used commands" so there's probably some change needed to the one
or the other to achieve consistency.

-Ben