Fix docstring for replaceable_entity.register_entities

[pacanada]

Motivation: I was trying to include a postgres view in the alembic context. When using alembic_utils.replaceable_entity.register_entities I realised that actually the description for the entity_types argument is not accurate: instead of List[str] should be List[Type[ReplaceableEntity]].

Example of usage: https://github.com/olirice/alembic_utils/blob/master/src/test/test_depends.py#:~:text=entity_types%3D%5B-,PGView,-%5D)

Changes:

• Update the docstring for register_entities

[olirice]

thank you