Use a more specific url for cvss qualitative severity system. #372

The url at is not specific.
https://github.com/nexB/vulnerablecode/blob/8cfb2bd5760d27259f425884926fec66a28798c2/vulnerabilities/severity_systems.py#L81

Use a more specific url https://www.first.org/cvss/specification-document#Qualitative-Severity-Rating-Scale instead