Typo in `InputError` raised by `Textractor` constructor

### Description
The constructor for the `Textractor` class requires that an argument be passed in for either `profile_name` or `region_name`. However, the `InputError` that's raised when both of these arguments are missing refers to the parameter as `region` instead of `region_name`:

https://github.com/aws-samples/amazon-textract-textractor/blob/f34c34a6c75dd4675642e52c1bad5a6c0eaf8ee4/textractor/textractor.py#L103-L105

This is a very minor inconsistency, but I think it can cause some confusion (it did for me)!