Update a docstring in readinput.py
Update a docstring in readinput.py

Removed a reference to a non-existent function `get_exposure_lazy`.

Former-commit-id: 573f7dc7f73f5279f5c2c28f5220d14573a994e0
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine