Fix empty link to PURL spec / VERS spec

[nathannaveen]

I was browsing through https://cyclonedx.org/docs/1.5/json/#vulnerabilities_items_affects_items_versions_items_range to gain a better understanding of CycloneDX version ranges. When I attempted to access the information via this link: https://github.com/package-url/purl-spec/VERSION-RANGE-SPEC.rst, it led me to a blank page. I believe the correct link should be https://github.com/package-url/purl-spec/blob/version-range-spec/VERSION-RANGE-SPEC.rst#version-range-specifier.

[jkowalleck]

duplicates #311 & related to #187