Trac #33307: Fix empty section display on prerequisites doc page

Prerequisites for building on many platforms
are listed at

- https://doc.sagemath.org/html/en/reference/spkg/_prereq.html

The empty homebrew section is not dealt with properly
and appears mixed with the next section, nix.

URL: https://trac.sagemath.org/33307
Reported by: slelievre
Ticket author(s): Matthias Koeppe
Reviewer(s): Samuel Lelièvre

build/bin/sage-spkg-info

@@ -75,6 +75,7 @@ for system in $systems; do
                 ;;
         esac
         sage-print-system-package-command $system --prompt='    $ ' --sudo install $system_packages
+        echo
 done
 if [ -z "$system" ]; then
     echo "(none known)"