Update broken link to problem package format

Also point to the legacy ICPC version for now. A new version is
planned to be released in September around the ICPC World Finals
in Astana, but until that's finalized, point to what we're actually
(roughly) implementing currently.

(cherry picked from commit 11fe36f)

Author: eldering

doc/manual/problem-format.rst

@@ -54,4 +54,4 @@ problem by uploading a zip file that contains only testcase files. Any jury
 solutions present will be automatically submitted when ``allow_submit`` is
 ``1`` and there's a team associated with the uploading user.
 
-.. _ICPC problem package specification: https://icpc.io/problem-package-format/spec/problem_package_format
+.. _ICPC problem package specification: https://icpc.io/problem-package-format/spec/legacy-icpc