[SE-0500] Fix proposal ID

Fixes:
```bash
curl https://download.swift.org/swift-evolution/v1/evolution.json 2>/dev/null | jq '.proposals[] | select(.id == "SE-500") | .errors'
[
  {
    "code": 0,
    "kind": "error",
    "message": "Proposal ID must include four decimal digits.",
    "suggestion": ""
  }
```

Author: MahdiBM

proposals/0500-package-manager-templates.md

@@ -1,6 +1,6 @@
 # Improving package creation with custom templates: SwiftPM Template Initialization
 
-* Proposal: [SE-500](0500-package-manager-templates.md)
+* Proposal: [SE-0500](0500-package-manager-templates.md)
 * Authors: [John Bute](https://github.com/johnbute)
 * Review Manager: [Franz Busch](https://github.com/FranzBusch)
 * Status: **Active Review (November 20 - December 4, 2025)**