fixed bug in _generate_rhs_docstring
fixed bug in _generate_rhs_docstring

template_values['specified_explanation'] should be equal to
```self._specifieds_doc_templates[self.specifieds_arg_type].format(**specified_template_values)```
and not
```self._specifieds_doc_templates[self.constants_arg_type].format(**specified_template_values)```

oliverlee/pydy,oliverlee/pydy,oliverlee/pydy