text: Title: Add the missing docstrings to the `spec_evaluator.py` file
  Body: Add the missing docstrings to the [spec_evaluator.py](https://github.com/scanapi/scanapi/blob/main/scanapi/evaluators/spec_evaluator.py) file
  
  [Here](https://github.com/scanapi/scanapi/wiki/First-Pull-Request#7-make-your-changes) you can find instructions of how we create the [docstrings](https://www.python.org/dev/peps/pep-0257/#what-is-a-docstring).
  
  Child of https://github.com/scanapi/scanapi/issues/411