Improve help command for oq prepare_site_model [skip CI]
Improve help command for oq prepare_site_model [skip CI]

Former-commit-id: eb46ee0e45b198d94234d822dbdccc8435d68a5f
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine