Defect link in the SBOM documentation

[CBeck-96]

The link "https://github.com/package-url/purl-spec/VERSION-RANGE-SPEC.rst" in vulnerabilities/affects/versions/range is defect.
An alternative might be "https://github.com/package-url/purl-spec/blob/version-range-spec/VERSION-RANGE-SPEC.rst".

[jkowalleck]

duplicates #187