Rename coefficent_1 to coefficient_1 in two_power_law

See #249.

Or fix it in `convert.py` as suggested in https://github.com/SasView/sasmodels/pull/438#issuecomment-756242605.

Maybe use the following rather than checking for version number:
```python
if name == 'two_power_law' and 'coefficent_1' in old_pars:
    old_pars['coefficient_1'] = old_pars['coefficent_1']
    del old_pars['coefficent_1']
```