fix(docs): fix Keys name of `JWT_AUTH`

It removes invalid `DEFAULT_` prefixes from the keys for `JWT_AUTH`.

Ref: https://github.com/GetBlimp/django-rest-framework-jwt/blob/master/rest_framework_jwt/settings.py#L10-L24
Merge pull request #8 from theskumar/patch-1

fix(docs): fix Keys name of `JWT_AUTH`