Update cluster.md
Update cluster.md

Former-commit-id: 3e5b2e4d2fe002e908397fc7b33e19a4e0c6c61c [formerly 3564cc18da27186ebe3e88aaf1f61912a1d2411c]
Former-commit-id: a29bcfec2c00b1a677d09a69d0335112bb8e73b5
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine