fix(k3s): fix mistakes in VEX and index files

[DmitriyLewen]

Description

Fix mistakes in k3s VEX and index documents.

Changes

• Use package ID without version in index.json (pkg:golang/github.com/k3s-io/k3s) (see https://github.com/aquasecurity/vex-repo-spec/blob/b795dc2403cc825956b6bb197c55734c1728f761/index.schema.json#L18)
• Add v prefix to k3s PURL versions (v1.34.1+k3s1)
• Fix typo: vulnerabiliy → vulnerability
• Add missing comma in JSON

Example

After these changes Trivy can suppress vulnerability:

Suppressed Vulnerabilities (Total: 1)
=====================================
┌──────────────────────────┬────────────────┬──────────┬──────────────┬───────────────────────────────────────────────────┬─────────────────────────────────────────────┐
│         Library          │ Vulnerability  │ Severity │    Status    │                     Statement                     │                   Source                    │
├──────────────────────────┼────────────────┼──────────┼──────────────┼───────────────────────────────────────────────────┼─────────────────────────────────────────────┤
│ github.com/docker/docker │ CVE-2025-54410 │ LOW      │ not_affected │ vulnerable_code_cannot_be_controlled_by_adversary │ VEX Repository: custom                      │
│                          │                │          │              │                                                   │ (https://github.com/ActiveState/advisories) │
└──────────────────────────┴────────────────┴──────────┴──────────────┴───────────────────────────────────────────────────┴─────────────────────────────────────────────┘