Broken policy documentation links in p.d.o

[Alfred Agrell]

Hello,

bug report: the [contrib] links on packages.debian.org currently lead to 
<https://www.debian.org/doc/debian-policy/#the-contrib-archive-area>, 
which is just an index page. The intended target has an extra 
ch-archive.html, 
<https://www.debian.org/doc/debian-policy/ch-archive.html#the-contrib-archive-area>.

This shows up on, for example, 
<https://packages.debian.org/stretch-backports/virtualbox> and 
<https://packages.debian.org/search?keywords=virtualbox>. The same issue 
exists on [non-free] packages too.

- Alfred Agrell