Update cluster.md
Update cluster.md

Former-commit-id: 2cbeb8652b2d2577411ac980029bb8d9b22131a2 [formerly e57296198e6ea5023ec1a5bd0986c1c7a55180e9]
Former-commit-id: ffd05ea6ade1ebbb50a702bcbda3573eb983231c
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine