#4880 - incorrect usage of elgg_get_entities_* parameters

Fixes http://trac.elgg.org/ticket/4880

- corrects correct singular usage of 'type' in elgg_get_entities_*
- corrects correct singular usage of 'subtype' in elgg_get_entities_*
- corrects correct usage of 'limit' in elgg_get_entities_* where limit doesn't matter