Server/Unix/p4/common/site/ReadMe.md: fill in the placeholder HMS
link (was literally "EDITME-UpdateThisURL", shipped in the 2026.1
GA tarball). Now points to https://swarm.workshop.perforce.com/projects/perforce_software-hms
matching the same link already used elsewhere (root README.md,
SDP_Legacy_Upgrades.Unix.adoc).
Doc-only, no tarball regeneration needed for this fix to take
effect on live Workshop browsing.