Fix download links: add missing /core/ in path

The 2.0.0 download links pointed to .../axis2/c/2.0.0/ but the
artifacts live at .../axis2/c/core/2.0.0/. This caused HTTP 404
on the source tarball, SHA-512, and ASC signature links.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>