swh-worker@.service: Fix --max-tasks-per-child celery flag

doc [1] says --max-tasks-per-child not --maxtasksperchild

• [1] http://docs.celeryproject.org/en/latest/userguide/workers.html#max-tasks-per-child-setting

Test Plan

octocatalog-diff

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Migrated from D1269 (view on Phabricator)