Minor Debian/Ubuntu install instruction update.

This is all a bit of a mess (and fragile and mostly their own fault),
but this at least gives people a clue. Fixes #16287. Patch from
bpeschier.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16647 bcc190cf-cafb-0310-a4f2-bffc1f526a37